Official Statistics

Surveillance of influenza and other seasonal respiratory viruses in the UK, winter 2023 to 2024

Updated 20 January 2025

First published: 5 June 2024

Corrected: 20 January 2025

This report was republished on 20 January 2025 to correct a typographical error describing the percentage of children aged under 5 years with RSV hospitalisations to SARI Watch sentinel trusts, that were aged under 1 year. The initially published figure of 647% has been corrected to 64.6%.

Main points

Influenza

Overall, most surveillance systems in the UK indicated lower influenza activity in the 2023 to 2024 season than in the 2022 to 2023 season.

Influenza activity in the 2023 to 2024 season was more prolonged than the 2022 to 2023 season, but peak activity was lower. Across most indicators cumulative burden estimates were lower in the 2023 to 2024 season than in the 2022 to 2023 season.

The influenza epidemic wave in the England, Scotland and Wales was characterised by an initial peak in activity in weeks 51 and 52 2023, followed by temporary decrease and subsequent increase in activity which peaked between weeks 4 and 5 2024. This activity pattern is most likely explained by a temporary decrease in social contacts and changes in healthcare seeking behaviour during the holidays. In Northern Ireland, there was a single peak in week 4 2024.

Influenza A predominated, with co-circulation of the A(H3N2) and A(H1N1)pdm09 subtypes. Influenza B type Victoria lineage circulated at low levels and became more prominent in 2024 as influenza A declined. All characterised viruses belonged to the same genetic clades as the vaccine strains.

Across nations, the influenza hospitalisation rate peaked between week 51 2023 and week 4 2024. Peak hospitalisation rates were lower than the previous influenza season.

Intensive care unit (ICU) or high dependency unit (HDU) influenza admissions across nations peaked between week 52 2023 and week 6 2024. Rates were lower than observed in the 2022 to 2023 season.

Modelling of influenza-attributable mortality in England estimated approximately 2,776 deaths due to influenza, compared to 15,465 in the previous season.

Provisional estimates of influenza vaccine effectiveness (VE) against laboratory confirmed influenza in primary care across age groups ranged between 46% and 54%. Effectiveness against hospitalisation ranged from 30% in those of age 65 and above to 74% in those between 2 and 17 years of age.

Across eligible groups, influenza vaccine uptake in the UK was generally lower in the 2023 to 2024 season compared to the 2022 to 2023 season.

Respiratory syncytial virus (RSV)

Overall RSV activity was consistent with the 2022 to 2023 season and recent pre-pandemic seasons.

RSV activity overall peaked around weeks 46 2023 (Scotland and Northern Ireland) to 48 2023 (England) across the UK. Several surveillance systems indicated a bimodal peak, with one peak in between weeks 46 and 48 driven by children below the age of 5, and a second peak between weeks 51 and 52 leading to hospitalisation rates peaking driven by adults aged 65 years and above.

Other non SARS-CoV-2 respiratory pathogens

Rhinovirus, parainfluenza, adenovirus and human metapneumovirus all showed activity consistent with expected seasonal patterns.

Background

Surveillance of influenza and other seasonal respiratory viruses in the 4 UK nations is undertaken throughout the year by teams within the UK Health Security Agency (UKHSA), Public Health Scotland (PHS), Public Health Wales (PHW) and the Public Health Agency (PHA) Northern Ireland, who are each responsible for monitoring influenza activity for their respective nation. UKHSA undertakes a combination of England and UK functions and collated this annual report in collaboration with the other public health agencies.

Outputs on influenza are normally published weekly during the winter season between October (week 40) and May (week 20), the period when influenza typically circulates. Since 2020 these reports have also covered COVID-19.

This report describes influenza activity observed in the UK in the period generally from week 40 2023 (week ending 8 October 2023) to week 14 2024 (week ending 7 April 2024). Data beyond this point is not considered so that contributors are certain the latest data points are not subject to reporting delays. Of note, there was some limited influenza B activity beyond week 14 2024 across the UK. Details can be found in the weekly reports for England, Scotland, Wales and Northern Ireland.

Devolved administrations are separately responsible for monitoring influenza with their own systems. Although systems across nations are very similar, the differences between these systems preclude combining data to a single national estimate. Surveillance of influenza and other respiratory viruses continues year-round with reduced reporting for weeks 21 to 39.

Community surveillance

Syndromic surveillance

England

In England, national UKHSA real-time syndromic surveillance systems include GP in-hours (GPIH) consultations and GP out-of-hours (GPOOH) contacts, emergency department (ED) attendances (Emergency Department Syndromic Surveillance System (EDSSS)) and NHS 111 calls and online assessments. These systems monitor a range of indicators sensitive to community influenza activity, for example NHS 111 ‘cold or flu’ calls and GP in-hours consultations for influenza-like illness (ILI). Coding and healthcare seeking behaviour may change over time and particular caution should be taken in comparing pre-pandemic, intra-pandemic and post-pandemic time periods.

Both clinical coding and healthcare-seeking behaviour have been affected by the COVID-19 pandemic. Syndromic data over the 2019 to 2020, 2020 to 2021 and 2021 to 2022 winter seasons should therefore be interpreted with some caution. Furthermore, during the 2022 to 2023 season, syndromic surveillance data should also be interpreted with some caution due to the ‘group A strep incident’ from week 48 2022. During this incident, national media reports of an increase in severe invasive group A streptococcus (iGAS) disease in children led to changes in healthcare-seeking behaviour across the NHS, particularly in children with GAS-type symptoms, which are likely to have been respiratory symptoms.

GPIH consultation data presented here represents the findings from a sentinel surveillance system, as reported throughout the most recent 6 influenza seasons. The levels (consultation rates) reported may differ from those presented in previous annual reports, which were based on a dual data feed system with a larger population coverage which ended in March 2021.

During the winter 2023 to 2024 season, GPIH ILI consultation rates increased from week 47 2023, peaking during week 51 2023 (7.3 consultations per 100,000 population) and again in week 5 2024 (8.6 consultations per 100,000 population). This peak at comparable time points with previous pre-pandemic years but was broadly lower than in those years (Figure 1). In the 2022 to 2023 season there was only one peak in week 51. Rates presented for weeks 52 and week 1 should be interpreted with caution as in those weeks there are less in-hour consultations due to the national holidays.

Figure 1. Weekly all age GP in-hours consultations for influenza-like illness (ILI), winter 2017 to 2024, England [note 1]

[note 1] Data from seasons 2020 to 2021 and 2021 to 2022 has been removed as there was low activity throughout these seasons.

Emergency department (ED) attendance syndromic surveillance data presented here includes 100 EDs (NHS type 1) that reported data throughout the most recent 6 influenza seasons. Therefore, numbers may differ slightly from those presented in previous annual reports, where a different number of EDs were included.

Acute respiratory infection (ARI) ED attendances increased gradually from week 41, peaked in week 52 2023 and again in weeks 4 to 6, and stabilized around 13,000 weekly attendances (Figure 2). The ARI indicator is a composite for respiratory diagnoses that follow below. Some caution is needed in interpreting attendances around week 52 2023 and week 1 2024 as increased attendances may be explained by less availability of primary care services due to bank holidays.

Figure 2. Weekly ED attendances for ARI by season, England

ILI ED attendances increased from week 48, showed 2 peaks in week 52 2023 and week 5 2024 and then gradually declined (Figure 3). Overall attendances were lower than in the previous season (17,535 attendances in the 2022 to 2023 season versus 10,873 attendances in the 2023 to 2024 season).

Figure 3. Weekly ED attendances for ILI by season, England

Pneumonia ED attendances gradually increased from week 40 2023, peaked in week 52 and stabilized at higher attendances than in previous seasons from week 3 2024 (Figure 4).

Figure 4. Weekly ED attendances for pneumonia by season, England

Acute bronchiolitis ED attendances were already rising in week 40 2023, peaked in week 47, decline steadily and stabilized around week 3 2024 (Figure 5).

Figure 5. Weekly ED attendances for acute bronchiolitis by season, England

In England, weekly 111 calls for ARI increased gradually in week 49 and peaked in week 52 2023. As with ED attendances, increases in weeks 52 and week 1 2024 should be interpreted with caution as patterns of health care use may change due to less availability of in-hour GP services (Figure 6).

Please note that recent updates to the NHS Pathways clinical tool used by NHS 111 have affected the reported levels of certain syndromic indicators. As a result of these changes, the cold and flu indicator has been removed from this report and replaced with a generic ‘acute respiratory infections’ calls indicator. The ‘acute respiratory infections’ indicator is based on a broad group of symptoms/provisional diagnoses that may be indicative of acute respiratory infections including, for example, influenza-like illness, otitis media, pharyngitis.

Figure 6. Weekly 111 calls for acute respiratory infection by season, England

Scotland

In Scotland, proportion of calls due to cold and flu increased from week 49 2023 and peaked in week 52 2023 and then gradually declined (Figure 7).

Figure 7. Percentages of calls to 111 for cold or flu by season, Scotland

Wales

In Wales, the proportion of nurse-triaged possible flu-related calls represents calls coded as either related to colds and flu, cough, sore throat, fever and/or headache. This metric showed increases from week 40 and peaked in week 52 at 27%, stabilising around 20% from week 2 2024 to week 14 2024.

Figure 8. Percentages of possible flu-related 111 calls by week, 2023 to 2024 season, Wales

SARS-CoV-2 Immunity and Reinfection Evaluation (SIREN) cohort

In SIREN 2.0 between September 2023 and March 2024, 5,970 healthcare workers from the original siren cohort completed fortnightly asymptomatic polymerase chain reaction (PCR) testing for SARS-CoV-2, influenza A/B and RSV, to monitor positivity rates and the emergence of new SARS-CoV-2 variants. In addition, just under 5,000 participants provided samples for serology testing at regular time points. Participants are distributed across the UK, with a median age of 53 years, 78% are female, 59% in clinical roles and 85% of white ethnicity.

Within the cohort, influenza positivity increased from week 48, peaked in week 52 2023 at 1.44 and again in week 5 2024 at 1.57% and decreased thereafter. RSV positivity increased from week 42, showing a temporary peak in activity in week 44 2023 at 1.39%, and after a few weeks of declining activity increases were observed which peaked at 2.27% positivity in week 51 2023 (Figure 9).

Figure 9. Weekly positivity for Influenza A/B and RSV, SIREN cohort, 2023 to 2024 season, UK

FluSurvey (internet-based surveillance)

FluSurvey, run by the UK Health Security Agency, is an internet-based participatory surveillance system based on the InfluenzaNet platform (an EU initiative with 12 participating countries). It is designed to monitor trends of influenza-like illness (ILI) in the community using self-reported respiratory symptoms from registered UK participants. The platform has been adapted to capture respiratory symptoms, exposure risk and healthcare seeking behaviours among participants to contribute to national surveillance of COVID-19 and influenza activity since week 44 of 2020.

Individuals aged 18 and over can register on the platform and complete a baseline profile questionnaire and weekly symptoms questionnaires on behalf of themselves or members of their household (referred to as ‘the participants’). The baseline profile questionnaire collects information on self-reported demographic, geographic, socioeconomic, and health data. Subsequently, participants are sent weekly reminders via email to report any symptoms relating to flu or COVID-19 that they may have experienced and their health-seeking behaviour as a result of their symptoms.

Recruitment of survey participants commenced from October 2023 on a new web platform with a mixture of previous participants and new participants. Therefore, the baseline demographics and level of symptoms may have changed compared with last season, including the possibility that new registrations may have been initiated by recent onset of illness. The number of participants will slightly differ from the national weekly surveillance report as these analyses are based on participants that completed both the profile and symptoms questionnaires and are deduplicated by week.

A total of 2,545 participants enrolled over the course of the season and completed at least one survey with an average weekly participation of 1,598 (62.8%), contributing each week.

There were more participants in the 65 and over year age group (45.5%) compared to other age groups (2.2% aged 0 to 18 years, 13.0% aged 19 to 45 years, 38.7% aged 46 to 64 years). There was a higher proportion of female participants compared to male participants (62.3% compared to 36.7%). The majority (2,233 or 87.7%) of participants were resident in England, with 162 participants in Scotland, 22 participants in Northern Ireland and 80 in Wales.

The European Centre for Disease Control (ECDC) ILI case definition of sudden onset of symptoms with at least one of fever (chills), malaise, headache, muscle pain and at least one of cough, sore throat, shortness of breath has been used for reporting. The ILI case definition is derived based on self-reported symptoms only among all those who participated during each reporting week. The proportion of weekly participants with self-reported ILI episodes peaked in week 50 at 5.3% compared to previous season which peaked in week 52 at 4.5% (Figure 10).

Healthcare use is presented as reported use of health services among persons who meet the ILI ECDC case definition. Where a person reports use of more than one health care service, secondary care will be indicated over primary care use and physical attendance to primary care will be indicated over use of remote services (for example, telephoning their GP or 111). Among participants who met the ILI ECDC case definition, the most frequently reported contact with healthcare services was a visit to their GP surgery.

Figure 10. Weekly ILI incidence per 1,000 participants and their rate and type of healthcare use reported through FluSurvey, by season

Crude self-reported daily social contact patterns by participants reporting any symptom are also shown (Figure 11), based on the number of people approached by the participants outside the household at less than one metre, on the day prior to survey completion. There was a slight increase in people reporting no social contacts around week 52 2023 consistent with the Christmas holiday period.

Figure 11. FluSurvey symptomatic participants’ self-reported number of social contacts outside the household, 2023 to 2024 season

FluDetector (internet-based surveillance)

UKHSA works with University College London (UCL) to assess the use of internet-based search queries as a surveillance method for ILI in England. This is part of work on early-warning surveillance systems for influenza, through the Engineering and Physical Sciences Research Council (EPSRC) Interdisciplinary Research Collaboration (IRC) project i-sense.

Combining natural language processing and machine learning techniques, a non-linear Gaussian process model was developed by UCL to produce real-time estimates of ILI. The supervised model, trained on historic national data from the Royal College of General Practitioners (RCGP) scheme, produces daily ILI estimates based on the proportion of ILI related search queries within a 10% to 15% sample of all queries issued, and is extracted daily from Google’s Health Trends Application Programming Interface.

In the 2023 to 2024 season, estimated rates peaked around weeks 50 and 52 in 2023 (both at a rate of around 8.0 per 100,000) and week 4 2024 (estimated rate of 10.2 per 100,000). There was also a slight increase in activity later in the season in week 12 (peaking at 4.1 per 100,000). Overall rates were lower than previous seasons with influenza activity (Figure 12).

Figure 12. Daily estimated ILI Google search query rates per 100,000 population by season

Acute respiratory infection incidents

ARI incidents in different settings that are reported to UKHSA health protection teams (HPTs) and entered onto the HPZone case and incident management system. Incidents are suspected or laboratory confirmed outbreaks and clusters of acute respiratory infections linked to a particular setting. All incidents are further investigated by the HPT in liaison with local partners. These data include only those reaching HPTs (rather than managed by health system partners such as community infection prevention and control teams) and classified as outbreaks rather than enquiries seeking public health advice.

The ARI definition includes presentations of both of ILI and other acute viral respiratory infections (AVRI). Causal pathogens can include influenza A and B, RSV, adenovirus, rhinovirus, parainfluenza, human metapneumovirus (hMPV) and SARS-CoV-2. Detections may reflect the accessibility of testing for a particular pathogen as well as the incidence of disease.

England

In England, there were a total of 2,051 ARI incidents in closed settings reported between week 40 of 2023 and week 14 of 2024. Of these, 1,661 (81.0%) were reported from care homes, 104 (5.1%) from educational settings, 215 (10.5%) from hospital settings, 21 (1.0%) from prisons and 50 (2.4%) from other settings. Where information on virological testing results was available, 708 (53.3%) were confirmed as SARS-CoV-2, followed by 472 influenza incidents (35.5%), 68 RSV incidents (5.1%) and 80 (6.0%) incidents linked to other respiratory viruses or mixed infections (Table 1).

Table 1. The number of incidents in England by institution and virus type between week 40 2023 and week 14 2024

Institution type	SARS-CoV-2	Influenza	RSV	Other respiratory viruses and mixed infections	Not available or not tested
Care home	519	383	51	63	645
Educational setting	27	18	9	2	48
Hospital	124	61	5	7	18
Prison	8	8	0	4	1
Other settings	30	2	3	4	11
Total	708	472	68	80	723

The majority of influenza outbreaks occurred in care home settings, with the highest number in a week observed during week 4 2024 (Figure 13). 438 of the influenza incidents were influenza A(not subtyped), 10 were influenza B, 2 were influenza A(H3), and 1 was influenza A(H1N1)pdm09. In 21 incidents it was unknown whether this involved influenza A or B.

Figure 13. The number of total influenza outbreaks by week and setting, the 2023 to 2024 season, England

Scotland

In Scotland, there were a total of 435 ARI outbreaks reported between week 40 2023 and week 14 2024, which was a decrease compared with 1,081 for the same time period in the 2022 to 2023 season. Of these, 344 (79.1%) were reported from care homes, 48 (11.0%) from educational settings, 36 (8.3%) from other settings, 5 (1.1%) from hospitals and 2 from prisons (0.5%).

Virological results indicate that 84 outbreaks were SARS-CoV-2, 45 were influenza A(not subtyped), 1 was influenza A(H1N1), 1 was influenza B, 13 were RSV, 6 were hMPV, 3 were rhinovirus, and 1 was seasonal coronavirus. The remaining 281 outbreaks did not have an organism reported.

Northern Ireland

In Northern Ireland, there were a total of 502 confirmed respiratory outbreaks reported to the Public Health Agency Acute Response Duty Room between week 40 2023 and week 14 2024, compared to a total of 762 outbreaks for the same period in the 2022 to 2023 season and 1,058 outbreaks in the 2021 to 2022 season. A total of 234 (46.6%) of outbreaks were reported in a care home facility, 231 (46.0%) in a hospital setting and 37 (7.4%) were reported from other settings.

Virological results were available for 502 confirmed respiratory outbreaks of which 420 were for SARS-CoV-2, 77 Influenza A (not subtyped), one influenza A(H1) and one influenza A(H3). Of the 77 influenza A (not subtyped) outbreaks reported, 8 facilities had another virus circulating at the same time. There were also 3 RSV outbreaks reported to the Public Health Agency (one of which had another virus circulating at the same time).

Further information on ARI incidents during the 2023 to 2024 season is available in the weekly influenza and COVID-19 surveillance report and information on ARI incidents occurring in previous seasons is available in previous annual flu reports.

Wales

In Wales, outbreaks are reported on a different interface (Tarian) from other nations. Therefore some caution is needed when comparing absolute numbers between nations.

There were a total of 205 ARI outbreaks reported between week 40 2023 and week 14 2024. Of these, 189 (92.2%) were reported from residential/care homes, 4 (11.0%) from educational settings, 5 (2.4%) from prisons and 7 (3.4%) from other settings. Virological results indicated that 173 outbreaks were due to SARS-CoV-2, 25 we due to influenza A(not subtyped), 1 was influenza A(H1N1), 1 was influenza B, 2 were due to RSV, 1 was due to parainfluenza and one was a mixed outbreak with influenza A(not subtyped) and RSV. In one outbreak no pathogen was identified.

Primary care surveillance

GP influenza-like-illness (ILI) consultations

England

Weekly rates of GP consultations for influenza-like illness (ILI) through the RCGP scheme remained below the 2023 to 2024 season moving epidemic method (MEM) baseline threshold of 10.25 per 100,000 for the duration of the influenza season (Figure 14).

The ILI rate for the 2023 to 2024 season was similar or lower than that observed in the 2022 to 2023 season up to week 2 2024. In the 2022 to 2023 season, the ILI rates were above the MEM baseline threshold levels for 6 weeks. GP ILI consultation rates for the 2023 to 2024 season were similar or below the 2017 to 2018, 2018 to 2019 and 2019 to 2020 seasons for the duration of the influenza season (Figure 14).

Figure 14. Weekly all age GP influenza-like illness rates by season, England (RCGP) [note 1]

See [note 1] as above.

Scotland

Weekly GP consultations for ILI surpassed the baseline MEM threshold of 11.7 per 100,000 in weeks 1, 5, and 8 to 9 2024 in Scotland. Overall, the ILI consultation rate was lower than levels observed in the 2022 to 2023 season, however raised levels of activity were observed over a longer period than the previous season (Figure 15).

Figure 15. Weekly all age GP influenza-like illness rates by season, Scotland [note 1]

See [note 1] as above.

Wales

Weekly GP consultations for ILI in Wales surpassed the baseline MEM threshold of 11 per 100,000 in weeks 4 and 5 2024 The ILI consultation rate remained within the baseline MEM threshold for the rest of the season. Overall seasonal ILI consultation was higher than levels observed in 2021 to 2022 but below the activity in 2017 to 2018 (Figure 16).

Figure 16. Weekly all age GP influenza-like illness rates by season, Wales [note 1]

See [note 1] as above.

Northern Ireland

Weekly GP consultations for ILI in Northern Ireland surpassed the baseline MEM threshold of 10.7 per 100,000 between week 1 2024 and week 8 2024 and peaked at 19.2 per 100,000 in week 4 2024 (Figure 17). Rates in the current season were lower than observed in the 2022 to 2023 season, however remained above the baseline threshold for a longer duration (8 weeks) than the previous season (6 weeks).

Figure 17. Weekly all age GP influenza-like illness rates by season, Northern Ireland [note 1]

See [note 1] as above.

Sentinel GP-based swabbing

England

In England, influenza positivity through the GP sentinel swabbing scheme in collaboration with the RCGP was lower overall in comparison with the previous season. Positivity began to increase in week 49 of 2023, compared to week 46 of 2022 last season (Figure 18). A total of 17,883 samples were tested between week 40 of 2023 and week 14 of 2024, 1,437 were positive for influenza. Between week 40 of 2022 and week 14 of 2023, a total of 9,062 samples were tested and 1,436 were positive for influenza. Influenza A(H1N1)pdm09 accounted for the majority of positive influenza specimens between week 40 2023 and week 14 2024. Among the positive samples for influenza, 53.5% (769 out of 1,437) were positive for influenza A(H1N1)pdm09 and 34.6% (497 out of 1,437) were positive for influenza A(H3N2).

Figure 18. Weekly positivity (%) for influenza in England by season, GP sentinel swabbing

Broader virological swab positivity is depicted in Figure 19. For non-flu pathogens detected through the RCGP sentinel swabbing scheme, rhinovirus was the most detected respiratory pathogen between week 40 2023 and week 14 2024 in England (28.3% of total positive specimens). Influenza comprised 17.8% of the total respiratory pathogens detected in the swabbing scheme. This was followed by RSV at 15.9% of total positive specimens, hMPV at 11.4%, SARS-CoV-2 at 10.1%, seasonal coronavirus at 9.1%, adenovirus at 3.5% and enterovirus at 3.9%.

Please note that starting in week 51, enterovirus and rhinovirus testing have been delayed.

Figure 19. Number of samples tested for SARS-CoV-2, influenza, and other respiratory viruses in England by week, 2023 to 2024 season, GP sentinel swabbing

Figure 20. Proportion of detections of SARS-CoV-2, influenza, and other respiratory viral strains among virologically positive respiratory surveillance samples in England by week, 2023 to 2024 season, GP sentinel swabbing scheme

Between week 40 2023 and week 14 2024, influenza positivity was highest in week 5 2024 in those aged under 5 years, in week 4 2024 in those aged between 5 and 17 years and in those aged between 18 and 64 years, and in week 6 2024 in those aged over 65 years (Figure 21).

Figure 21. Weekly positivity (%) for influenza by age group in England, GP sentinel swabbing, 2023 to 2024 season [note 2]

[note 2] The highlighted line corresponds to the age group in the subplot title, grey lines correspond to all other age groups.

Scotland

In Scotland, the Community Acute Respiratory Infection (CARI) surveillance programme is the GP sentinel surveillance system and tests for a range of respiratory pathogens: SARS-CoV-2, influenza A and B, RSV, adenovirus, coronavirus (non-SARS-CoV-2), hMPV, rhinovirus, parainfluenza and Mycoplasma pneumoniae. Between week 40 of 2023 and week 14 of 2024, 13.0% (2,786 out of 21,467) of total detections were influenza: 4.9% (1,048 out of 21,467) were influenza A(H3N2), 4.2% (908 out of 21,467) were influenza A(H1N1)pdm09, 2.0% (439 out of 21,467) were influenza A(not subtyped), 1.7% (358 out of 21,467) were influenza B. Additionally there were 10 were co-infections (<0.1% of total detections) of influenza A(H1N1)pdm09 and influenza A(H3N2) and 23 were co-infections of influenza A and influenza B (0.1% of total detections).

For non-flu pathogens, rhinovirus was the most commonly detected respiratory pathogen through sentinel sources (CARI) for the 2023 to 2024 season (up to week 14, 2024), with 17.7% of 20,535 samples testing positive for rhinovirus. Mycoplasma pneumoniae was the second most common non-influenza pathogen (8.7% of 20,575 samples) followed by seasonal coronavirus (non-SARS-CoV-2) (5.4% of 20,468 samples), RSV (6.2% of 20,469 samples), hMPV (6.0% of 20,469 samples), SARS-CoV-2 (5.2% of 21,727 samples), parainfluenza (4.0% of 20,440 samples) and adenovirus (2.2% of 20,447 samples). Figure 22 below includes individual pathogens and total samples received, in addition to coinfections as some samples tested positive for more than one pathogen (8.2% of all samples).

Figure 22. Number of CARI samples and positives by pathogen, Scotland, week 40 2023 to week 14 2024

Wales

In Wales, 5,259 samples were received for testing from sentinel GP practices between week 40 2023 and week 14 2024. Of these, 504 specimens tested positive for influenza during the 2023 to 2024 season through GP sentinel swabbing: 139 were influenza A(H3N2), 200 were influenza A(H1N1)pdm09, 74 were influenza A(not subtyped) and 91 were influenza B. Of non-flu respiratory pathogens, 650 samples tested positive for Rhinovirus, 346 for Mycoplasma Pneumoniae, 274 for RSV, 274 for SARS-CoV-2, 254 for hMPV, 220 for other coronaviruses, 174 for parainfluenza, 144 were positive for enterovirus, 101 for Adenovirus and 29 were positive for Bocavirus.

Figure 23. Number of GP sentinel samples and positives by pathogen, Wales, week 40 2023 to week 14 2024

Northern Ireland

In Northern Ireland, 668 swabs were received from week 40 2023 to week 14 2024. 188 specimens tested positive for influenza during the 2023 to 2024 season through GP sentinel swabbing, of which 135 tested positive for influenza A(H3), 42 for influenza A(H1), 3 were influenza A(not subtyped) and 8 were influenza B. The highest amount of positive samples were received in week 4 2024 and a majority of positive samples (140) were received between weeks 2 2024 and week 8 2024. 16 swabs tested positive for RSV, with most detections (11 positive samples) occurring between weeks 44 and 47 2023.

Secondary care surveillance

Influenza hospitalisations

England

In England, the Severe Acute Respiratory Infection (SARI) Watch surveillance system was established in 2020 to report the number of laboratory-confirmed influenza, COVID-19 and RSV cases admitted to hospital and critical care units (ICU or HDU) in acute NHS trusts. This replaced surveillance systems used in previous seasons for reporting influenza and COVID-19 cases admitted to hospital – the UK Severe Influenza Surveillance System (USISS) and the COVID-19 Hospitalisations in England Surveillance System (CHESS). Aggregate level data is submitted weekly by acute NHS trusts in England. A week is based on the ISO week system running from Monday to Sunday. There are 137 acute NHS trusts in England at this time.

The weekly rate of new admissions of influenza cases is based on the catchment population of those NHS trusts who made a return in that week. Surveillance is usually between week 40 (around October) to week 20 (late May) in the following year. However, during the pandemic (affecting seasons 2020 to 2021 and 2021 to 2022), influenza and RSV surveillance were extended to week 39 (comprising one-year surveillance) to detect out of season rebounds. Influenza ICU or HDU surveillance was also extended to week 39 2023 in the 2022 to 2023 season.

Trends in influenza hospitalisation and critical care admission should be interpreted in the context of testing practices for acute respiratory infections. In recent years there has been wider implementation of rapid molecular point of care tests for influenza in hospital settings. From a public health surveillance perspective, it is important to consider a step change in influenza case ascertainment in more recent years.

Subtyping of influenza A admissions to critical care is strongly recommended in line with existing guidance for hospital settings. This is in the context of vigilance over avian influenza and concern that severe cases presenting in ICU or HDU settings may be missed. In November 2023, a human case of A(H1N2)v was detected in the Yorkshire region which prompted a reminder to support subtyping of influenza A cases in the region. The testing direction may impact on the ratio of subtyped to unsubtyped cases in surveillance data. Furthermore, the ratios of different flu types and subtypes should be interpreted with caution due to possible differences in the availability of assays for different subtypes.

The cumulative rates presented are based on a sum of weekly rates which take into account only trusts participating in that week. Figures 24, 27, 32 and 35 were adjusted to account for the leap week in 2020.

Through SARI Watch, a total of 8,026 hospitalised confirmed influenza cases were reported by 31 participating sentinel NHS acute trusts in England from week 40 2023 to week 14 2024 inclusive. Of 31 trusts, 26 were regular reporters participating in 18 weeks or more in this period. This compares with 9,713 cases in the previous season (from week 40 2022 to week 14 in 2023) reported by 44 trusts.

As a sentinel surveillance system, the number of trusts volunteering to participate may vary between seasons. Therefore, rates of hospitalised influenza cases are presented here to compare between different seasons.

The cumulative admission rate between week 40 and week 14 was 76.9 per 100,000 trust catchment population in the 2023 to 2024 season. This compares with cumulative admission rates of between week 40 to week 14 of 94.2 per 100,000 in the 2022 to 2023 season.

In the 2023 to 2024 season, the baseline MEM threshold was breached in week 49 2023 with a rate of 2.20 per 100,000 (Figure 24). Two peaks followed, one in week 52 (week commencing December 25) 2023 with a rate of 6.65 per 100,000 and another in week 4 (week commencing 22 January) 2024 with a rate of 7.55 per 100,000, both crossing into the medium impact range. The epidemic wave in the 2023 to 2024 season was similar to those in pre-pandemic seasons in terms of timing of the peak and magnitude of rates.

In comparison, in the 2022 to 2023 season the rate breached the baseline threshold in week 45 2022 with a rate of 1.67 per 100,000. The peak was in week 51 with a rate of 18.09 per 100,000, crossing into the very high impact range. The epidemic wave in the 2022 to 2023 season was characterised by earlier activity, a sharp increase, a high peak and steep decreases (please refer to the previous annual report for further details).

Weekly confirmed influenza hospitalisation rates to sentinel hospital trusts in England since winter 2016 are shown in Figure 27.

Figure 24. Weekly influenza hospital admission rates per 100,000 trust catchment population (with MEM thresholds), reported through SARI Watch sentinel surveillance, England [note 1]

See [note 1] as above.

Figure 25 shows the proportional distribution of 8,026 sentinel influenza hospital admissions reported up to week 14 2024 by subtype/type and age group. Overall, 1,156 were influenza A(H1N1)pdm09, 543 were influenza A(H3N2), 5,770 were influenza A(not subtyped) and 557 were influenza B.

Influenza B can become a more common cause of admission later in the season than influenza A. In the 2023 to 2024 season, small increases in influenza B hospitalisations were observed from January 2024 peaking at 0.48 per 100,000 (based on 50 cases) in week 9 2024 but did not overtake influenza A hospitalisations. In the previous season influenza B also emerged later in the season but in higher case numbers, peaking at 1.23 per 100,000 (based on 116 cases) in week 7 2023 and overtaking influenza A. In both seasons the late season rise in influenza B mainly affected those aged 15 to 44 years.

The subtyping of influenza A data is used for public health surveillance and allows a more complete picture of circulating subtypes affecting severe cases. Hospital trusts are encouraged to perform influenza subtyping locally where possible or regionally and to integrate results into local information systems.

This sentinel system has noted a high proportion of influenza A that has not been subtyped in recent years. This may reflect increase in the use of rapid nucleic acid amplification tests (NAATs) used at the point of care as well as longer standing use of laboratory NAATs that do not include subtyping. Overall, of total influenza A hospitalised cases in the 2023 to 2024 season, 77% (5,770 out of 7,469) were not subtyped, ranging from 62% (69 out of 111) in 6 to 11 months to 82% (649 out of 794) in those aged 1 to 4 years. The data shows that those under one year had a lower proportion of influenza A samples subtyped at 68% for under 6 months and 6 to 11 months groups combined (165 out of 242). For other age groups, this percentage ranged from 76% to 79% apart from 1 to 4 years. As comparison, in the previous season, from week 40 2022 to week 14 2023, the overall proportion of influenza A hospitalised cases that were not subtyped was higher at 85% (7,164 out of 8,422).

The ratio of influenza A(H1N1)pdm09 to influenza A(H3N2) is a useful indicator of which of circulating influenza A subtype is dominant. This ratio was 1.1 in the early part from week 40 to 47 2023 in favour of influenza A(H1N1)pdm09 and increased to 2.2 in the latter part of the season. The increase may reflect the choice of assay that prioritised detection of influenza A(H1N1)pdm09 over influenza A(H3N2). This coincided with the direction to NHS trusts in late November 2023 to subtype influenza A samples following the detection of a human case of influenza A(H1N2)v. As a result of testing changes from November 2023, it may be difficult to definitively conclude what was the true main circulating subtype among hospitalised influenza cases in the 2023 to 2024 season.

Figure 25 shows the proportion of influenza subtypes by age group amongst hospitalised influenza cases in the 2023 to 2024 season. There was a slightly higher proportion of influenza B cases in younger age groups. The highest numbers were among those aged 15 to 44 years (n=1,801), followed by those aged 75 to 84 years (n=1,172) and 85 years (n=908). Absolute counts per age group will be published in the accompanying datafile.

However, the rates which measure impact in the context of a susceptible population, were highest in the elderly particularly those aged 85 years and over (Figure 26). The rate in this age group peaked at 46.11 per 100,000 in week 4 (commencing 22 January) 2024. The rate in 75 to 84 years was the next highest, peaking also in week 4 2024 at 21.21 per 100,000. The rate in under 5 years was the next highest peaking at 17.74 per 100,000 also in week 4 2024. The rate in 15 to 44 years was one of the lowest peaking at 3.91 per 100,000 in week 5 2024.

Figure 25. Proportion of influenza hospital admissions by influenza type and subtype, and age group, reported through SARI Watch sentinel surveillance, England, week 40 2023 to week 14 2024

Figure 26. Weekly influenza hospital admission rate by age group, reported through SARI-Watch sentinel surveillance, week 40 2023 to week 14 2024 [note 2]

See [note 2] as above.

Figure 27. Weekly all-age rate of confirmed influenza hospital admissions to sentinel trusts, 2016 to 2024, England

Scotland

Scottish influenza hospital admissions are calculated by linking the Rapid Preliminary Inpatient Dataset (RAPID) emergency or non-injury hospital admissions to positive Electronic Communication of Surveillance in Scotland (ECOSS) test results. RAPID captures all hospital admissions. The ECOSS data set provides all laboratory test data for respiratory viruses (including SARS-CoV-2, influenza, and RSV) in Scotland. Linkage is performed by linking the Community Health Index (CHI) number with the admission date to the ECOSS test results and test date.

Cases counted in the hospital admission totals for these pathogens are defined as those admitted to hospital, appearing in RAPID and testing positive for influenza (appearing in ECOSS with a test or specimen date) within 14 days prior or 48 hours post RAPID admission date.

There were a total 4,911 influenza hospital admissions reported in Scotland between week 40 2023 and week 14 2024, yielding a cumulative rate of 89.6 per 100,000 compared to 121.7 per 100,000 in the 2022 to 2023 season. Influenza hospital admissions peaked in week 52 2023 and again week 4 2024. There was a relatively equal ratio of influenza A(H1N1)pdm09 (n=448) admissions and influenza A(H3N2) admissions (n=444) during this period, with a late-season surge of influenza B admissions from week 8 2024 onwards. Further evolution of these trends can be found in the PHS weekly report.

Figure 28. Weekly influenza hospitalisation rates, by season, Scotland

Figure 29. Weekly number of influenza admissions by influenza subtype, week 40 2023 to week 14 2024, Scotland

Northern Ireland

Influenza admissions data comes from all trusts providing healthcare (100% coverage). Any admission with a positive influenza test taken with a period between 7 days prior to admissions, anytime during the admission and up to 7 days after discharge is included. Admission activity increased in week 48, showed a single peak in week 4 2024 at 15.9 per 100,000 and steadily declined for the remainder of the season. As there is 100% coverage counts by influenza subtype are shown in Figure 30. There was a total of 2,708 admissions, with a cumulative admission rate of 141.7 per 100,000. Overall, 314 admissions were influenza A(H1N1)pdm09, 1,050 were influenza A(H3N2), 1,282 were influenza A(not subtyped) and 62 were influenza B. Influenza A(H3N2) was dominant in all age groups (Figure 31).

Figure 30. Weekly number of influenza admissions by influenza subtype, week 40 2023 to week 14 2024, Northern Ireland

Figure 31. Proportion of influenza hospital admissions by influenza subtype and age group, Northern Ireland, week 40 2023 to week 14 2024

Wales

Hospitalized ARI cases in Wales are calculated by linking hospital admissions from Patient Administration Systems (PAS) to Datastore test results using the patient NHS number. Cases are defined as those admitted to hospital who tested positive for an ARI within 28 days prior to admission or up to day 2 of an inpatient stay (where admission date is day 1).

A total of 1,159 hospitalized influenza cases were reported in Wales from week ending 40 2023 to week 13 2024. Activity increased in week 46 2023 and peaked in week 52 2023 at 2.2 per 100,000 and again in week 4 2024 at 3.4 per 100,000 (Figure 32).

Figure 32. Weekly influenza hospitalisation rate per 100,000, Wales, the 2023 to 2024 season

Influenza ICU or HDU admissions

England

Through SARI Watch, a total of 873 critical care (ICU or HDU) confirmed influenza admissions were reported by 112 NHS acute trusts across England from week 40 2023 to week 14 2024. This compares with 1,660 critical care influenza admissions reported by 110 trusts from week 40 2022 to week 14 in 2023.

The cumulative rate was 1.92 per 100,000 trust catchment population based on data from week 40 2023 to week 14 2024. In the 2022 to 2023 season the cumulative rate for the same reporting interval was 3.78 per 100,000.

The ICU or HDU admission rate for influenza crossed the baseline threshold in week 51 2023 with rate of 0.12 per 100,000 trust catchment population (Figure 32). An initial small peak followed in week 52 at 0.12 per 100,000. Two subsequent relatively larger peaks occurred in week 4 and 6 2024, both at 0.20 per 100,000 and remaining in the low impact range.

As comparison, in the 2022 to 2023 season, the ICU or HDU admission rate for confirmed influenza increased earlier compared with pre-pandemic seasons crossing the baseline threshold in week 46 2022 with rate of 0.12 per 100,000. The peak was in week 51 2022 at 0.64 per 100,000, crossing into the high impact range for the first time since the 2019 to 2020 season. In the context of all the recent seasons (except 2020 to 2021 and 2021 to 2022), the peak in the rate of ICU or HDU admission for influenza in the 2023 to 2024 season was lower compared to other seasons with influenza activity (Figure 32).

Of 873 influenza ICU or HDU admissions reported up to week 14 2024, 116 were influenza A(H1N1)pdm09, 37 were influenza A(H3N2), 676 were influenza A (not subtyped) and 44 were influenza B (Figure 33). There was no observed increase in influenza B admissions later in the season.

A high proportion of influenza A detections remains unsubtyped. Of all influenza A ICU or HDU cases in the 2023 to 2024 season, 82% (676 out of 829) were not subtyped, ranging from 73% (52 out of 71) in 5 to 14 years to 91% (20 out of 22) infants aged 6 months or less (Figure 34). The overall proportion of unsubtyped influenza A cases in critical care was similar in the 2022 to 2023 season at 85% (1,196 out of 1,459).

Figure 33 shows the distribution of influenza ICU or HDU case numbers by age group and influenza type and subtype in the 2023 to 2024 season. The highest numbers were among those aged 15 to 44 years (n=175).

However, the rate of admission was highest in older adults and those under 5 years (Figure 34) although rates across the board were low (less than 0.6 per 100,000) due to the low number of cases. The highest rate was for under 5 years at 0.54 per 100,000 in week 6 but the rates were mainly highest in 45 to 54 and 55 to 64 years. The rates for those aged 85 years and over was one of the lowest reflecting critical care admission protocols.

The surveillance of influenza ICU or HDU admissions includes a separate collection for fatalities (any cause) among influenza cases admitted in ICU or HDU as an indication of severity near real time for use in conjunction with other severity indicators such as excess mortality – this is not equivalent to cause of death as included in death registrations. There were 75 fatalities reported among influenza cases admitted to ICU or HDU from week 40 2023 to week 14 2024. Of these, 12 were influenza A(H1N1)pdm09, 2 were influenza A(H3N2), 58 were influenza A(not subtyped) and 3 were influenza B. Patients aged 65 years or over made up the largest group in ICU and HDU influenza fatal case reporting (40% of fatal cases). In the season the 2022 to 2023 season there were 139 fatalities among influenza cases admitted to ICU or HDU.

Weekly confirmed influenza ICU or HDU admission rates in England since winter 2016 are shown in Figure 35.

Figure 33. Weekly influenza ICU or HDU admission rates per 100,000 trust catchment population (with MEM thresholds), reported through SARI Watch mandatory surveillance, England [note 1]

See [note 1] as above.

Figure 34. Proportion of influenza ICU or HDU admissions by influenza subtype/type and age group, reported through SARI Watch mandatory surveillance, England, week 40 2023 to week 14 2024

Figure 35. Weekly influenza ICU or HDU admission rate by age group, reported through SARI Watch mandatory surveillance, week 40 2023 to week 14 2024 [note 2]

See [note 2] as above.

Figure 36. Weekly all-age rate of confirmed influenza ICU or HDU admissions, 2016 to 2024, England

Scotland

Patients admitted to ICU or HDU with recently confirmed influenza are identified from the Scottish Intensive Care Society Audit Group (SICSAG) that collects detailed patient level data on all patients in ICU or HDU across Scotland. Additional information on patients with laboratory confirmed influenza is collected through ICU enhanced surveillance forms. All patients that are admitted to ICU or HDU with a positive influenza test result within a period of 14 days before the ICU or HDU admission and the ICU or HDU discharge date are included. Where the discharge date is missing, any patient with a positive influenza test result within a period of 14 days before ICU or HDU admission and 7 days after ICU or HDU admission are included.

There was a total of 240 ICU admissions in adults in the period between week 40 2023 and week 14 2024. Of these, 38 were positive for influenza A(H1N1)pdm09, 26 were positive for influenza A(H3N2), 170 were positive for influenza A(unknown subtype) and 6 were positive for influenza B. Admissions peaked in week 52 with 25 admissions (Figure 36). The age distribution and subtypes are shown in Figure 37.

Figure 37. Weekly ICU or HDU admissions by influenza subtype, Scotland, the 2023 to 2024 season

Figure 38. Proportion of influenza ICU or HDU admissions by influenza type and subtype, and age group, reported through SICSAG, Scotland, week 40 2023 to week 14 2024

Wales

75 influenza ICU admissions were reported in Wales.

Among 25 admissions were subtyping information was available, 14 were positive for influenza A(H1N1)pdm09, 6 were positive for influenza A(H3N2), and 5 were positive for Influenza B.

Northern Ireland

Critical care data from Northern Ireland was not available.

ECMO admissions

UKHSA collects data on every adult patient admitted to a severe respiratory failure (SRF) centre, for extra corporeal membrane oxygenation (ECMO) or other advanced respiratory support, whether or not the primary cause is known to be infection related. There are 7 SRF centres in the UK (6 in England and 1 in Scotland) participating in the UKHSA ECMO surveillance module. Surveillance is all year round.

For the surveillance, SRFs can select from a range of principal causes of admission, including test confirmed acute respiratory infection (ARI) and non-infection cause (such as asthma, primary cardiac and trauma. Causative pathogen is reported for test confirmed ARI admissions.

Between week 40 2023 and week 14 2024, there were 94 admissions to SRF centres requiring ECMO in the UK (Figure 38). Of these, confirmed ARI accounted for 38% (36 out of 94) and non-infection accounted for 46% (43 out of 94). Suspected ARI and sepsis of non-respiratory origin accounted for 9% and 7% of admissions requiring ECMO respectively but underlying numbers were small.

Of 36 ARI ECMO admissions in the 2023 to 2024 season, 16 were for influenza (9 influenza A (not subtyped), 4 influenza A(H3N2), 3 influenza A(H1N1)pdm09 and 0 influenza B cases). In this period, there were 2 COVID-19 admissions, but no RSV admissions.

In the previous season (from week 40 2022 to week 14 2023), there were 113 admissions to SRF centres requiring ECMO in the UK. Of these, confirmed ARI accounted for 58% (65 out of 113) and non-infection accounted for 32% (36 out of 113). Of 65 confirmed ARI cases, 39 were influenza (19 influenza A (not subtyped), 8 influenza A(H3N2), 5 influenza A(H1N1)pdm09 and 7 influenza B cases). There were 3 RSV admissions. In pre-pandemic seasons, there were 37 influenza admissions to adult SRF centres in 2019 to 2020, 110 in 2018 to 2019 and 62 in 2017 to 2018.

Figure 39. Laboratory confirmed ECMO admissions in adults (influenza and non-influenza confirmed) to Severe Respiratory Failure centres in the UK, week 14 2022 to week 14 2024

RSV

England

UKHSA collates data on confirmed hospitalised RSV cases in England through the SARI Watch surveillance system. For RSV, this is a sentinel surveillance system.

Between week 40 2023 and week 14 2024, a total of 3,016 confirmed RSV cases (2,758 hospitalised to lower level of care and 258 admitted to ICU or HDU) were reported from 24 participating sentinel trusts. Of 24 trusts, 18 were regular reporters (participating in 18 weeks or more in the 27-week reporting period). The cumulative admission rate between week 40 2023 and week 14 2024 was 37.79 per 100,000 trust catchment population. This compares to 32.98 per 100,000 in the 2022 to 2023 season for the same 27-week interval from the 2022 to 2023 season.

The epidemic wave for RSV in the 2023 to 2024 season and the 2022 to 2023 season (both up to week 14) was similar to those in pre-pandemic seasons in terms of timing of the peak and seasonality. However, RSV activity in the 2022 to 2023 season appeared to be lower even after a return to seasonality which may be partly explained by disease activity in earlier weeks (from week 21 to 39 2022) as RSV transitioned towards normal seasonality following easing of pandemic control measures. It should be noted that the displacement effect was strongest in summer of 2021 (please refer to the previous annual report for details).

The overall RSV hospitalisation rate (inclusive of ICU or HDU admissions) in the 2023 to 2024 season peaked at 3.95 per 100,000 trust catchment population in week 48 2023. In the 2022 to 2023 season this peaked at 3.17 per 100,000 in week 47 2022. For comparison, peak weekly admission rates for RSV (lower level of care and ICU or HDU) were observed at 5.28 per 100,000 in 2019 to 2020, 4.67 per 100,000 in 2018 to 2019 and 4.06 per 100,000 in 2017 to 2018 (Figure 39).

Figure 40. Weekly hospitalised RSV case rate per 100,000 trust catchment population, England, post pandemic (the 2023 to 2024 season and the 2022 to 2023 season) and pre- pandemic seasons (2019 to 2020, 2018 to 2018 and 2017 to 2018)

Children under the age of 5 years are most affected by RSV with substantially high hospitalisation rates. Those aged 65 years and over have the next highest burden.

In the 2023 to 2024 season the RSV hospitalisation rate for children aged under 5 years peaked at 44.83 per 100,000 in week 48 2023 (Figure 40). The epidemic curve in the 2023 to 2024 season and the 2022 to 2023 season for those under 5 years both followed a typical seasonal pattern. This is in contrast to the 2 preceding seasons over the pandemic (2020 to 2021 and 2021 to 2022) where displacement of RSV activity to the summer following the easing of COVID-19 restrictions (thus allowing some other respiratory viruses to circulate). The previous annual report provides a more detailed description. 

As comparison to pre-pandemic years, the RSV hospitalisation rate in those under 5 year olds peaked at 56.73 per 100,000 in week 51 in 2019 to 2020, 67.37 per 100,000 in week 48 in 2018 to 2019, and at 56.38 per 100,000 in week 46 in 2017 to 2018.

In the 2023 to 2024 season from week 40 to week 14 inclusive, there were 1,739 RSV hospitalisations (lower level of care or ICU or HDU admissions) to SARI Watch sentinel trusts in those aged under 5 years. Of these, 64.6% (1,123) were in children aged under 1 year, including 849 aged 6 months or younger.

Figure 41. Rate of RSV hospitalisation in those aged under 5 years per 100,000 trust catchment population by week of admission and season, sentinel data from acute NHS trusts, England, post pandemic (the 2023 to 2024 season and the 2022 to 2023 season) and pre- pandemic seasons (2019 to 2020, 2018 to 2018 and 2017 to 2018)

The rate for those aged 65 years and over peaked at 6.43 per 100,000 in week 52 2023, later than the peak among children aged under 5 years (Figure 41). For those aged 85 years and over the rate peaked at 17.26 per 100,000 in week 52 2023. In pre-pandemic seasons the rate for the elderly (65 years and over) was 6.93 per 100,000 in 2019 to 2020, 6.07 per 100,000 in 2018 to 2019 and 3.77 per 100,000 in 2017 to 2018. These also peaked later than the peaks for children aged under 5 years. Rates across all age groups are shown in Figure 42.

Figure 42. Weekly hospitalisation (including ICU or HDU) admission rates by age group for RSV cases reported through SARI Watch sentinel surveillance, England

Scotland

Scottish RSV hospital admissions are calculated by linking the Rapid Preliminary Inpatient Dataset (RAPID) emergency or non-injury hospital admissions to positive ECOSS test results. RAPID captures all hospital admissions. The ECOSS data set provides all laboratory test data for respiratory viruses (including SARS-CoV-2, influenza, and RSV) in Scotland. Linkage is performed by linking the Community Health Index (CHI) number with the admission date to the ECOSS test results and test date.

Cases counted in the hospital admission totals for these pathogens are defined as those admitted to hospital, appearing in RAPID and testing positive (appearing in ECOSS with a test/specimen date) within 14 days prior or 48 hours post RAPID admission date.

There was a total of 4,057 RSV admissions reported in Scotland from week 40 2023 to week 14 2024, yielding a cumulative hospital admission rate of 74.0 per 100,000. Activity peaked in week 46 2023 at 394 admissions (7.2 per 100,000) and declined thereafter (Figure 42).

Figure 43. Rate of overall RSV hospitalisation per 100,000 population by week of admission, Scotland, by season

Northern Ireland

There was a total of 1,284 RSV admissions reported in Northern Ireland from week 40 2023 to week 14 2024, yielding a cumulative hospital admission rate of 67.2 per 100,000. Activity peaked in week 47 2023 at 147 admissions (7.7 per 100,000) and declined thereafter (Figure 44).

Figure 44. Rate of overall RSV hospitalisation per 100,000 population by week of admission, Northern Ireland, the 2023 to 2024 season

Wales

Hospitalized ARI cases in Wales are calculated by linking hospital admissions from Patient Administration Systems (PAS) to Datastore test results using the patient NHS number. Cases are defined as those admitted to hospital who tested positive for an ARI within 28 days prior to admission or up to day 2 of an inpatient stay (where admission date is day 1).

A total of 1,386 hospitalized RSV cases were reported in Wales from week 40 2023 to week 13 2024. Activity was increasing in week 40 and peaked in week 44 at 3.4 per 100,000 and steadily declined thereafter (Figure 45).

Figure 45. Weekly RSV hospitalisation rate per 100,000, Wales, the 2023 to 2024 season

Microbiological surveillance

Influenza

England

The Respiratory Datamart system began during the 2009 influenza pandemic to collate all laboratory testing information in England. It is now used as a sentinel laboratory surveillance tool, monitoring all major respiratory viruses in England. 16 laboratories in England, including 4 public health laboratories, 11 NHS hospital laboratories and a UKHSA national laboratory, reported data for this season. The majority of samples were received from hospitals.

Overall influenza positivity showed an increased level of activity (above 5%) between week 49 of 2023 and week 11 of 2024, with peak positivity (17.5%) much lower than that seen in the 2022 to 2023 season (31.0%) (Figure 46). There was bimodal activity with a first lower peak in week 52 2023 and the main peak seen in week 4 2024. Reduced mixing during the holiday period may explain this pattern. The majority of detections were influenza A (Figure 47). Of the detected influenza A viruses that were subtyped, the majority were influenza A(H3N2). Influenza B positivity increased to levels between 1.6% and 1.7% between week 5 2024 and week 11 2024.

Comparatively, the 2023 to 2024 season was a longer, lower, later season than the 2022 to 2023 season detections of all influenza through Respiratory DataMart. Peak positivity was also lower than the 25.3% in 2019 to 2020 and 28.6% in 2018 to 2019 season. However, positivity estimates should be interpreted with caution as they arise from time periods when SARS-CoV-2 was not co-circulating.

Figure 46. Weekly influenza swab percentage positivity through Respiratory Datamart sentinel laboratories by season, England

See [note 1] as above.

Figure 47. Number of influenza detections by subtype through Respiratory Datamart sentinel laboratories in England

The weekly count of confirmed cases by each major type or subtype of influenza through Respiratory DataMart from winter 2009 onwards is shown in Figure 48.

Figure 48. Weekly number of influenza detections by subtype and overall percentage positivity, Respiratory Datamart, England, 2010 to 2024

Scotland

In Scotland, overall influenza positivity is reported through non-sentinel sources via ECOSS. There have been fewer numbers of influenza diagnoses recorded to date this season with a total of 9,791 samples testing positive for influenza between week 40 of 2023 and week 14 of 2024 when compared to the previous season. Overall influenza positivity peaked between week 52 2023 and week 4 2024 at 14.9% and 14.7% respectively (figures 49 and 50).

Figure 49. Weekly ECOSS influenza positivity from week 40 of 2023 to week 14 of 2024, Scotland

Figure 50. Weekly ECOSS influenza counts by influenza subtype from week 40 of 2023 to week 14 of 2024, Scotland

Northern Ireland

4,601 (9.5% of 48,258 samples) samples tested positive in Northern Ireland during the 2023 to 2024 season. Positivity increased starting in week 49 2023 and peaked once in week 3 2024 at approximately 22% and then gradually declined (Figure 48).

Figure 51. Weekly influenza positivity by season, Northern Ireland

A total of 2,322 of these samples were influenza A(not subtyped), 1,675 were influenza A(H3), 490 were influenza A(H1) and 114 were influenza B. Distribution of these samples across the season are shown in Figure 52.

Figure 52. Weekly positive influenza samples by subtype, Northern Ireland

Wales

Diagnostic virology test results comprise test result for diagnostic respiratory panel testing in patients with ARI symptoms. The vast majority of these patients are in hospital, with a small proportion from non-sentinel community sources.

Out of 33,162 samples, 2,304 tested positive for influenza A, and 251 tested positive for influenza B. Overall influenza positivity started to increase in week 46 2023 and peaked in week 52 2023 at 11.8% and again in week 5 2024 at 17.0% (Figure 53).

Figure 53. Weekly influenza positivity, diagnostic virology, Wales

Respiratory syncytial virus (RSV)

England

From week 40 2023 to week 14 2024, RSV detections reported through the Respiratory DataMart surveillance system in England showed a similar seasonality as in pre-COVID pandemic seasons. All-age positivity peaked at 12.5% in week 48 2023. This compares with positivity peak of 13.4% in 2019 to 2020 (Figure 54). However, positivity estimates should be interpreted with caution as they arise from time periods when SARS-CoV-2 was not co-circulating. Positivity by age group for the 2023 to 2024 season is shown in Figure 55 with highest positivity in those aged under 5 years old.

Surveillance of RSV is ongoing with further data of current trends published in the weekly national influenza and COVID-19 surveillance report.

Figure 54. Weekly overall RSV swab percentage positivity through Respiratory DataMart by season, England [note 1]

See [note 1] as above.

Figure 55. Respiratory DataMart weekly positivity (%) for RSV by age, England

Scotland

In Scotland, RSV was the most commonly detected non-influenza respiratory pathogen detected through ECOSS for the 2023 to 2024 season (up to week 14 2024), accounting for 22.2% of all positive samples.

Northern Ireland

Across the whole season 1,605 (5.4% of 29,611 samples) samples tested positive for RSV. RSV positivity peaked in week 46 2023 at 18.3%, and then gradually declined.

Wales

Across the whole season 2,271 (6.8% of 33,162 samples) samples tested positive for RSV. RSV positivity peaked in week 44 2023 at 27%, and then gradually declined.

Other seasonal respiratory viruses

England

Of the other respiratory viruses monitored through the respiratory DataMart system, the highest levels of positivity were observed with rhinovirus throughout the season. Rhinovirus positivity was highest at the beginning of the season and then decreased and fluctuated between 6% and 24% positivity through the rest of the season, with lower activity levels seen between week 2 2024 and week 10 2024 (Figure 56a).

Consistent with typical previous seasons, low levels of adenovirus were observed throughout the season (Figure 56b). Parainfluenza positivity started to increase slowly from week 47 2023, and by the data cut-off time for this report (week 14 2024) it increased to 7.2% in week 14 which was slightly higher than the 2022 to 2023 season peak of 6.3% in week 13 2023 (Figure 56c). Parainfluenza subtype analysis showed that parainfluenza 3 was the main circulating subtype accounting for 74.4% of all 4 subtypes in this season with data up to week 14 2024, which was similar to the previous seasons between 2020 and 2023 but different from the 2019 to 2020 season when parainfluenza 1 accounted for 36.0% followed by parainfluenza 3 (25.7%), parainfluenza 2 (22.1%) and parainfluenza 4 (16.2%).

Human metapneumovirus (hMPV) activity in the 2023 to 2024 season was generally consistent with recent seasons outside of those impacted by the COVID-19 pandemic and peaked at 4.3% in week 52 2023, compared with the peak of 5.2% in week 52 2022 in the 2022 to 2023 season (Figure 56d).

Figure 56a. Weekly sample positivity for adenovirus by season, DataMart sentinel laboratories, England

Figure 56b. Weekly sample positivity for parainfluenza by season, DataMart sentinel laboratories, England

Figure 56c. Weekly sample positivity for rhinovirus by season, DataMart sentinel laboratories, England

Figure 56d. Weekly sample positivity for human metapneumovirus (hMPV) by season, DataMart sentinel laboratories, England

Scotland

Rhinovirus was the second-most commonly detected non-influenza respiratory pathogen detected through non-sentinel sources (ECOSS) for the 2023 to 2024 season (up to week 14, 2024), contributing to 15.9% of positive samples. Followed by Mycoplasma pneumoniae (5.3% of positive samples), hMPV (4.8% of positive samples), adenovirus (4.4% of positive samples), parainfluenza (3.6% of positive samples) and seasonal coronavirus (3.5% of positive samples).

Wales

In Wales, of the 33,162 testing episodes in hospital patients that had a sample collected from a hospital setting, the most commonly detected non-influenza respiratory pathogens were SARS-CoV-2 (7.8%, 2,574 out of 33,162) and rhinovirus (13.1%, 4,351 out of 33,162).

Other detected causes of respiratory infection included: adenovirus (4.2%, 1,412 out of 33,162), enterovirus (2.7%, 882 out of 33,162), seasonal coronavirus (2.3%, 773 out of 33,162), hMPV (4.2%, 1,403 out of 33,162), parainfluenza (2.2%, 729 out of 33,162) and Mycoplasma pneumoniae (2.9%, 953 out of 33,162).

Northern Ireland

Only data regarding influenza and RSV was available.

Preceding, co- and secondary respiratory infections with influenza

The surveillance of preceding, co-, and secondary infections with COVID-19 in England began as part of UKHSA’s response to the COVID-19 pandemic and was expanded to include the surveillance of pathogens acquired in conjunction with influenza in 2022. Regular reporting in the National flu and COVID-19 surveillance report began in late 2020. The workstream aims to provide rapid detection and timely surveillance of bacterial, viral, and fungal pathogens acquired in conjunction with COVID-19 and influenza.

Seasonal respiratory virus co-infections are defined as a patient with laboratory-confirmed influenza and a sample testing positive for another respiratory viral organism within one day (before or after) of the influenza positive specimen date. Other respiratory viral organisms of interest are adenovirus, SARS-CoV-2, enterovirus, human metapneumovirus, parainfluenza, respiratory syncytial virus (RSV), rhinovirus, and seasonal coronavirus. Full preceding, co-, and secondary infection definitions are available in Appendix 1 of the National flu and COVID-19 surveillance graphs packs.

Preceding and secondary infection definitions for respiratory viral infections are defined as the positive specimen date occurring 2 to 27 days pre- or post-laboratory-confirmed influenza positive specimen date. Please note that undertesting of pathogens may result in an underestimate of preceding, co-, and secondary infection cases. Furthermore, children receiving the live attenuated Influenza vaccine (LAIV) in the 14 days prior to swabbing may result in positive influenza detection in the absence of infection.

Seasonal respiratory viral pathogens acquired in conjunction with influenza

Please note, the base infection is any type of influenza (A, B, or both) for all viral preceding, co-, and secondary infections, except for influenza B where the base infection is influenza A.

A total of 4,216 pre-, co-, and secondary infections were detected across all seasonal respiratory viruses amongst persons with influenza in the 2023 to 2024 season in England, via the Respiratory DataMart and SGSS systems. Of which, 2,943 were defined as co-infections, 733 as preceding, and 540 as secondary to an influenza infection (Figure 57).

Between week 40 2023 and week 14 2024, 215 respiratory viral co-infections of influenza B and influenza A were identified in England. A total of 8 preceding and 13 secondary influenza B infections with influenza A were reported (Figure 58).

Between week 40 2023 and week 14 2024, the most frequent viral organisms (preceding, co-, or secondary infections) identified from respiratory specimens amongst persons with influenza were SARS-CoV-2, respiratory syncytial virus (RSV), and rhinovirus (Figure 55). SARS-CoV-2 was the most commonly detected pre-, co-, and secondary infection specimen, accounting for 47.0% of all pre-, co-, and secondary infections, followed by RSV, which accounted for 13.6%.

Figure 57. Weekly number of respiratory viral specimens, by timing of diagnosis, in persons with influenza diagnosed in England between week 40 2023 and week 14 2024

Figure 58. Most frequent respiratory viral specimens, by timing of diagnosis, in persons with influenza diagnosed in England between week 40 2023 and week 14 2024, [note 3]

[note 3]: The baseline infection is any type of influenza (influenza A or B or both) for all viral preceding/co-/secondary infections except for influenza B where the baseline infection is influenza A.

Virus characterisation

England

UKHSA characterises the properties of influenza viruses through one or more tests, including genome sequencing (genetic analysis) and haemagglutination inhibition (HI) assays (antigenic analysis). This data is used to compare how similar the currently circulating influenza viruses are to the strains included in seasonal influenza vaccines, and to monitor for changes in circulating influenza viruses. The interpretation of genetic and antigenic data sources is complex due to a number of factors, for example, not all viruses can be cultivated in sufficient quantity for antigenic characterisation, so that viruses with sequence information may not be able to be antigenically characterised as well. Occasionally, this can lead to a biased view of the properties of circulating viruses, as the viruses which can be recovered and analysed antigenically may not be fully representative of majority variants, and genetic characterisation data does not always predict the antigenic characterisation.

Between week 40 2023 and week 16 2024, the UKHSA Respiratory Virus Unit have genetically characterised, by sequencing of the haemagglutinin (HA) gene, 2,072 influenza A viruses (925 A(H3N2) and 1,147 A(H1N1)pdm09 viruses) and 434 influenza B viruses.

The 925 influenza A(H3N2) viruses genetically characterised all belong in the genetic clade 2 (formerly known as 3C.2a1b.2a.2). The Northern Hemisphere 2023 to 2024 season influenza Sequencing of the haemagglutinin (HA) gene shows that these influenza A(H3N2) viruses belong in clade 2a.3; defined by the amino acid changes clade 2a plus D53N, N96S (+CHO), and I192F in the HA gene. Two viruses have the additional HA substitution E50K (clade 2a.3a). Approximately 99.8% of characterised A(H3N2) viruses have the additional HA substitutions I140K and I223V (clade 2a.3a.1). A(H3N2) vaccine strain (an A/Darwin/9/2021-like virus) also belongs in genetic clade 2 but not the 2a.3a.1 subclade.

The 1,147 influenza A(H1N1)pdm09 viruses characterised to date this season, all belong in genetic clade 5a.2 (formerly known as 6B.1A.5a.2). The Northern Hemisphere 2023 to 2024 season influenza A(H1N1)pdm09 vaccine strain (an A/Victoria/2570/2019-like virus) also belongs in this genetic clade. 86% of viruses characterised by sequencing of the haemagglutinin (HA) gene possessed the additional amino acid substitutions of clade 5a.2 plus K54Q, A186T, E224A, R259K and K308R (clade 5a.2a). 14% of characterised A(H1N1)pdm09 viruses have the additional HA substitutions P137S, K142R, D260E, T277A (subclade 5a.2a.1). 138 (86.8%) of the 5a.2a.1 viruses have also had the additional HA substitution T216A.

The 434 influenza B/Victoria lineage viruses that have been genetically characterised, all belonging in clade V1A.3a.2. The Northern Hemisphere 2023 to 2024 season influenza B/Victoria lineage vaccine strain (a B/Austria/1359417/2021-like virus) also belongs in this clade.

The first confirmed human case of influenza A(H1N2)v in England was detected in November 2023 through the RCGP sentinel surveillance scheme. Whole genome sequencing was undertaken directly from human clinical respiratory material and the sequence was consistent with a swine H1N2 virus belonging to 1B.1.1 HA clade. The neuraminidase (N2) and the internal gene segments all demonstrated a very close relationship to contemporary H1N2 swine influenza A viruses circulating in Great Britain with no evidence of a new reassortment. UKHSA conducted a rapid technical assessment and contact tracing, as well as enhanced surveillance in the York and Humber region, (by increased sentinel GP swabbing in the affected region and analysis of influenza A-positive samples from local hospitals). No further cases of A(H1N2)v were detected.

The Respiratory Virus Unit has confirmed by genome sequencing the detection of LAIV viruses in 5 influenza A positive samples and 8 influenza B positive samples collected since week 40 2023, all from children aged between 2 and 16 years of age, consistent with known shedding characteristics of LAIV viruses as detailed in laboratory guidance.

The WHO consultation on influenza vaccine composition for the Northern Hemisphere 2024 to 2025 season took place in February 2024. The recommendation is the use of trivalent vaccines for the use in the 2024 to 2025 northern hemisphere season, as follows.

Egg-based vaccines:

• A/Victoria/4897/2022 (H1N1)pdm09-like virus
• A/Thailand/8/2022 (H3N2)-like virus (updated from previous composition)
• B/Austria/1359417/2021 (B/Victoria lineage)-like virus

Cell culture- or recombinant-based vaccines:

• A/Wisconsin/67/2022 (H1N1)pdm09-like virus
• A/Massachusetts/18/2022 (H3N2)-like virus (updated from previous composition)
• B/Austria/1359417/2021 (B/Victoria lineage)-like virus

Scotland

Virus characterisation in Scotland is performed by the West of Scotland Specialist Virology Centre (WoSSVC) throughout the influenza season on a small number of influenza samples. Between week 40 2023 and week 14 2024, the 116 influenza A(H1N1)pdm09 viruses characterised all belong in genetic clade 5a.2 (formerly known as 6B.1A.5a.2); 54 belong to the Norway/25089/2022 strain and 62 to the Sydney/5/2021strain. The Northern Hemisphere the 2023 to 2024 season influenza A(H1N1)pdm09 vaccine strain (an A/Victoria/2570/2019-like virus) also belongs in this genetic clade.

Between week 40 2023 and week 14 2024, the 88 influenza A(H3N2) viruses genetically characterised in Scotland, all belong in genetic clade 2 (formerly known as 3C.2a1b.2a.2), specifically the A/Darwin/9/2021 strain. A(H3N2) vaccine strain (an A/Darwin/9/2021-like virus) also belongs in genetic clade 2.

There were 18 influenza B/Victoria lineage viruses genetically characterised, all belonging in clade V1A.3a.2. The Northern Hemisphere the 2023 to 2024 season influenza B/Victoria lineage vaccine strain (a B/Austria/1359417/2021-like virus) also belongs in this clade.

Influenza antiviral susceptibility

Influenza positive samples are genome sequenced and screened for mutations in the virus neuraminidase (NA) and the cap-dependent endonuclease (PA) genes known to confer neuraminidase inhibitor (NAI) or baloxavir marboxil resistance, respectively. The samples tested are routinely obtained for surveillance purposes, but diagnostic testing of patients suspected to be infected with antiviral-resistant virus is also performed.

Influenza virus sequences from samples collected between weeks 40 2023 and 16 2024 have been analysed. Analysis of 897 A(H3N2) viruses by sequencing did not identify any oseltamivir resistant virus. Of the 1,144 A(H1N1)pdm09 NA sequences analysed by sequencing, 6 oseltamivir resistant viruses were found. These were taken from 5 patients:

• Patient 1: 2 samples with H275Y – immune compromised adult patient known to have received oseltamivir treatment
• Patient 2: one sample with H275Y – adult patient with COPD exacerbation known to have received oseltamivir treatment
• Patient 3: one sample with H275Y – immune compromised adult patient known to have received oseltamivir treatment
• Patient 4: one sample with D199E – immune compromised adult patient known to have received oseltamivir treatment
• Patient 5: one sample with H275Y – adult patient referred for resistance testing, clinical details pending

Other mutations in the NA (I223V, S247N, I223V+S247N) known to cause low level reduction of sensitivity to NAIs were detected in several influenza A(H1N1)pdm09 viruses (Table 2). The clinical relevance of these mutations is under investigation.

One influenza B virus with an amino acid change (I221T) known to confer slightly reduced sensitivity to oseltamivir and peramivir was detected. There were no known markers of resistance to NAIs detected in further 431 influenza B NA sequences analysed.

No viruses with known markers of resistance to baloxavir marboxil were detected in 799 A(H3N2), 941 A(H1N1)pdm09 and 370 influenza B PA sequences analysed. There were 2 influenza A(H3N2) viruses identified with a deletion of amino acids 37 to 38 and one influenza A (H1N1)pdm09 virus with a deletion of amino acids 35 to 45; virus isolation and further characterisation is underway. A schematic overview is given in Table 2.

Table 2. Antiviral susceptibility of influenza positive samples tested at UKHSA Respiratory Virus Unit

Influenza subtype	Sequences analysed	H275Y [note 4]	D199E [note 4]	I221T	I223V	S247N	I223V + S274N
A(H1N1) pdm09	1,144	5	1	0	2	22	6
A(H3N2)	897	0	0	0	0	0	0
B/Victoria	432	0	0	1	0	0	0

[note 4]: mutations documented to clinically result in reduced susceptibility to neuraminidase inhibitor Oseltamivir.

Mortality

Influenza-attributable deaths

The FluMOMO model has been used by the UK Health Security Agency (UKHSA) for many years to estimate influenza-related mortality, adjusting for extreme temperature, and is published in annual reports. However, with the COVID-19 pandemic leading to very large mortality levels from late March 2020, and with very little influenza circulation in 2020 to 2021 and 2021 to 2022 winters, models were not run in those years. For the 2022 to 2023 season, a new model was developed to incorporate COVID-19, details of which are given in the 2023 working paper.

In the 2023 to 2024 season the new FluMOMO model was run using data from week 40 2012 to week 13 2024. Further details of the model for this season, with comparisons to the 2023 model are given in the 2024 working paper. Briefly, the model estimates attributable mortality based on the pattern of all cause deaths by week compared to that seen for influenza activity, extreme cold or heat and COVID-19 death certifications. It allows an estimate of attributable mortality irrespective of whether deaths are recorded as due to these factors.

During the 2023 to 2024 season, influenza circulated at fairly low levels from mid-December 2023 to early March 2024. COVID-19 also circulated with waves throughout the period. There were periods of extreme cold with alerts issued. In 3 weeks, the mean weekly temperature was below 3°C, the threshold used in the FluMOMO model. 

Figure 59 represents the weekly number of all-age deaths and attribution to influenza, COVID-19 and extreme temperature covering the most recent 2 winters (week 40 2022 to 13 2024). The model demonstrates that in the winter of the 2023 to 2024 season excess mortality was fairly low with the greatest contributors on top of the baseline (grey) being COVID-19 (blue), followed by influenza (red) and cold weather (green) (Figure 59, Table 3 and Table 4).

Influenza-related mortality for winter the 2023 to 2024 season is estimated at about 2,800 which is much lower than that seen in the 2022 to 2023 season and lower than all the pre-pandemic years in the model except 2013 to 2014. The adapted method is less suited to estimation for cold and the more recent Omicron COVID-19-related mortality as they tend not to cause sharp mortality spikes, but they are included and give a higher estimate of COVID-19 attributable mortality (approximately 5,500) than influenza for the season.

For temperature-related mortality, the 1,400 estimate is based on 3 cold weeks and is lower than the 2022 to 2023 season which had some colder weeks than the 2023 to 2024 season. This estimate is contingent on the definition of cold used and only considers extreme cold rather than general cold which is captured in the baseline component of the model along with other seasonal factors not specifically included in the model.

Figure 59. Weekly number of all-age deaths (black line) and attribution to influenza (red bars), COVID-19 (blue bars) and cold weather (green bars), England, week 40 2022 to week 13 2024

Table 3. Estimated number of deaths associated with influenza, by age observed through the adapted FluMOMO algorithm with 95% confidence intervals [note 5], England, the 2023 to 2024 season (week 40 2023 to week 13, 2024) [note 6]

Age	Total estimate	Lower 95% CI	Upper 95% CI
0 to 4 years [note 5]	87	73	102
5 to 14 years	23	16	31
15 to 64 years	188	156	222
65 years and over	2,478	2,321	2,639
Total	2,776	2,613	2,939

[note 5]: Confidence intervals do not include uncertainty from model specification.

[note 6]: Estimates in children should be treated with caution, as the method is not calibrated to estimate such small excesses.

Table 4. Estimated number of all age deaths associated with influenza, COVID-19 and cold weather observed through the adapted FluMOMO algorithm, England, 2012 to 2013 season to the 2023 to 2024 season [note 7]

Season	Influenza	COVID-19	Cold	Unexplained [note 8]	Total
2012 to 2013	14,788	0	4,125	4,239	23,152
2013 to 2014	1,412	0	0	-5,341	-3,929
2014 to 2015	30,703	0	1,046	-638	31,112
2015 to 2016	13,091	0	63	861	14,014
2016 to 2017	19,244	0	432	2,912	22,588
2017 to 2018	26,557	0	2,302	5,599	34,457
2018 to 2019	5,670	0	999	-4306	2,364
2019 to 2020	8,583	[note 9]	0	53,923	62,506
2020 to 2021	[note 10]	[note 10]	[note 10]	[note 10]	[note 10]
2021 to 2022	442	29,058	0	-4,840	24,660
2022 to 2023	15,465	14,077	3,963	2,954	36,457
2023 to 2024	2,776	5,457	1,438	-37	9,635

[note 7]: Year is week 40 of one year to week 20 of the next except 2023 to 2024 which ends on week 13.

[note 8]: Unexplained is negative if the estimated excess from influenza, COVID-19 and cold weather is more than the observed total excess above the baseline.

[note 9]: COVID-19 not included in the model for 2019 to 2020. Many of the unexplained deaths that year will be due to COVID-19.

[note 10]: Season 2020 to 2021 not included in the model.

Vaccination

Seasonal influenza vaccine uptake in adults

Although all countries of the UK use standardised specifications to extract vaccine uptake data from IT information systems in primary care (GP system suppliers), there are some differences in extraction specifications, so comparisons between nations should be made with caution.

England

In England, the uptake of seasonal influenza vaccine is monitored by UKHSA throughout the season based on weekly and monthly extracts from GP system suppliers via ImmForm for the cohorts primarily delivered via the GP practice.

Cumulative uptake on influenza vaccinations administered up to 29 February 2024 was reported from 96.9% (6,152 out of 6,346) of GP practices in England in the 2023 to 2024 season.

Comparative data is up to 28 February 2023 where vaccine uptake was reported from 97.1% (6,257 out of 6,447) of GP practices in England in the 2022 to 2023 season.

This season saw a vaccine uptake of 77.8% in those aged 65 years and over (compared with 79.9% in the 2022 to 2023 season ) and 41.4% for those aged 6 months to under 65 years of age with one or more underlying clinical risk factors (excluding pregnant women without other risk factors and carers), compared with 49.1% in the 2022 to 2023 season. Vaccine uptake in pregnant women was 32.1%, compared with 35.0% in the 2022 to 2023 season (Table 5a).

Scotland

In Scotland, the uptake of seasonal influenza vaccine is estimated by PHS throughout the season, using a combination of aggregated data returns submitted by staff in all the territorial NHS boards on a weekly basis for those eligible in the following cohorts: 6 months to 2 years at risk, 2 to 5 years not at school, primary and secondary school pupils, and weekly extracts of data downloaded from the national clinical data store (NCDS) which contains individual-level data for adults eligible for flu vaccine, and for one NHS board, the data relating to the childhood schools’ and nurseries’ programmes. Please note pregnancy uptake data is not available for 2023 to 24.

By the end of week 14, 2024 a total of 1,598,869 influenza vaccines were delivered to adults currently resident in Scotland corresponding to a 53.7% uptake. Among adults aged 65 years and older, 79.8% have been vaccinated against influenza during the current vaccination programme. Among at risk groups, 42.2% have been vaccinated against influenza. Figures (numerators and denominators) are representative of the current living Scottish population of each respective eligible cohort. Deaths and leavers from Scotland have been removed.

Data is presented in Scotland’s weekly national respiratory report.

Wales

In Wales, the uptake of seasonal influenza vaccine is monitored on a weekly basis by Public Health Wales throughout the season based on automated weekly extracts of Read coded data using software installed in all General Practices through the Audit+ Data Quality System.

Cumulative uptake data on influenza vaccinations administered were received from 100% of GP practices in Wales in the 2023 to 2024 season. This showed a vaccine uptake of 72.5% in the 65 years and over age group (compared to 76.3% in the 2022 to 2023 season ) and 39.1% for those aged 6 months to under 65 years of age with 1 or more underlying clinical risk factors, compared to 44.2% in the 2022 to 2023 season.

Overall uptake in pregnant women was 60.9% compared to 60.0% in the 2022 to 2023 season. In Wales, vaccine coverage in pregnant women is measured using a survey of pregnant women giving birth each year during January. In addition, as elsewhere in the UK, data are also automatically collected from general practices for women with pregnancy related Read codes, these data report uptakes of 47.0% in pregnant women at risk and 33.7% in healthy pregnant women, however issues of denominator inflation in these measures mean that the figures are likely to be underestimates.

Northern Ireland

In Northern Ireland, the uptake of seasonal influenza vaccine is monitored by the Public Health Agency (PHA) of Northern Ireland. From 2021 to 2022 onwards influenza vaccine uptake has been determined using data extracted from regional Immunisation Information System developed by the Department of Health (DoH) Digital team, known as the Vaccine Management System (VMS). Caution should be used when considering influenza vaccination uptake rates from 2021 to 2022 in comparison to previous seasons, due to the introduction of the new VMS involving new methods of recording and extracting influenza vaccine data.

In the population aged 65 years and older, uptake was 78.0% (compared to 83.0% in the 2022 to 2023 season ). Uptake was 31.1% in the population of 50-64 year olds in the 2023 to 2024 season, compared to 54.5% in the 2022 to 2023 season. Uptake figures may be subject to revision with improvements in data collection and reporting.

Healthcare workers

England

In England, vaccine uptake among all frontline healthcare workers was 43.1% (Trusts and GP practices). In Trusts, vaccine uptake was 42.8% (from 93.6% of Trusts responding), a decrease from 49.9% vaccine uptake in the 2022 to 2023 season. In GP practice, vaccine uptake was 61.8% (from 9.9% of GP practices), a decrease from 66.3% vaccine uptake in the 2022 to 2023 season.

Scotland

In Scotland, in the 2023 to 2024 season, the combined uptake among health and social care workers was 42.2%, a decrease from 55.7% vaccine uptake in the 2022 to 2023 season.

Wales

In Wales, uptake reached 40.5% in the 2023 to 2024 season compared to 46.7% in the 2022 to 2023 season.

Northern Ireland

In Northern Ireland, uptake in frontline healthcare workers excluding social care was 24.8% in 2023 to 24. Uptake figures may be subject to revision with improvements in data collection and reporting.

Vaccine uptake by country tables

Table 5a. Vaccine uptake in target groups in the UK: England, 2023 to 2024 season

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	8,608,243	11,061,423	77.8
6 months to under 65 years at-risk	3,908,092	9,445,565	41.4
Pregnant - No risk	173,683	577,561	30.1
Pregnant - At-risk	51,755	124,998	41.4
Pregnant - All	225,438	702,559	32.1
Frontline Healthcare Workers [note 11]	516,582	1,197,259	43.1

Table 5b. Vaccine uptake in target groups in the UK: Scotland, 2023 to 2024 season

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	913,409	1,144,785	79.8
18 years to under 65 years at-risk	390,872	925,769	42.2
Pregnant - no risk	[note 12]	[note12]	[note 12]
Pregnant - at-risk	[note 12]	[note 12]	[note12]
Pregnant - all	[note 12]	[note 12]	[note 12]
Frontline healthcare workers [note 11]	75,546	179,015	42.2

Table 5c. Vaccine uptake in target groups in the UK: Wales, 2023 to 2024 season

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	509,224	702,643	72.5
6 months to under 65 years at-risk	182,773	467,149	39.1
Pregnant - no risk	5,474	16,233	33.7
Pregnant - at-risk	1,113	2,367	47.0
Pregnant – all [note 13]	221	363	60.9
Frontline healthcare workers [note 11]	26,947	66,534	40.5

Table 5d. Vaccine uptake in target groups in the UK: Northern Ireland, 2023 to 2024 season

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	256,703	329,235	78.0
50 to under 64 years [note 14]	108,155	348,104	31.1
18 years to under 50 years at-risk [note 15]	15,632	131,851	11.9
Pregnant - no risk	[note 12]	[note 12]	[note 12]
Pregnant - at-risk	[note 12]	[note 12]	[note 12]
Pregnant - all	[note 12]	[note 12]	[note 12]
Frontline healthcare workers [note 11]	10,024	41,156	24.8

[note 11]: Excludes social care workers for England, Scotland, Wales and Northern Ireland.

[note 12]: Complete data from Scotland and Northern Ireland on pregnancy is not available at the time of reporting.

[note 13]: Taken from the annual point of delivery survey. The 2023 to 2024 survey was based on a sample of 363 women delivering over a 5-day period in January 2024.

[note 14]: In 2023 to 2024, the offer of vaccination was extended to all aged 50 to 64 years from January 2024 onwards.

[note 15]: In 2023 to 2024 a denominator was only available for those aged 18 to 64 years at risk.

Influenza vaccine programme for children

England

The influenza vaccine uptake in 2 and 3 year olds in England is monitored by UKHSA throughout the season, through weekly and monthly extracts from GP system suppliers via ImmForm. Cumulative vaccine uptake on influenza vaccinations administered up to 29 February 2024 was reported from 93.6% (5,934 out of 6,342) of GP practices in England in the 2023 to 2024 season.

Comparative data is up to 28 February 2023 where vaccine uptake was reported from 98.4% (6,339 out of 6,442) of GP practices in England in the 2022 to 2023 season. This season saw a vaccine uptake for all GP-registered 2 year olds of 44.1% (compared with 42.3% in the 2022 to 2023 season) and was 44.6% in 3 year olds (compared with 45.1% in the 2022 to 2023 season ) in England.

The combined uptake for 2 and 3 year olds was 44.4% compared with 43.7% in the 2022 to 2023 season.

In the 2023 to 2024 season, school-aged children in year groups reception to year 11 (aged 4 to 16 years) were eligible for the influenza vaccine programme. There were differences in eligibility for the school-aged programme between the 2023 to 2024 season and the 2022 to 2023 season, as last season Years 10 and 11 were vaccinated subject to vaccine availability, and at a national level very little activity took place in these years groups.

The programme was mainly delivered via a school-based route, with one area (Isle of Scilly) delivering vaccinations through general practice. Vaccine uptake was monitored through manual returns by local teams for their responsible population.

An estimated 4,022,141 out of 8,063,277 eligible children in school years reception to year 11 in England received at least one dose of influenza vaccine during the period 1 September 2023 to 31 January 2024. The overall uptake was 49.9%. Vaccine uptake in all primary school age children (age 4 to 11 years old) was 55.1% compared with 56.3% in the 2022 to 2023 season (Table 6). Vaccine uptake in secondary school age children (age 11 to 16 years old) was 42.8% and due to differences in eligibility, data is not comparable to last season (Table 6).

Vaccine uptake in children of school age generally decreases with increasing age. This trend has been seen in the previous seasons (Table 6 and Figure 60).

Table 6. Percentage influenza vaccine uptake in children of school age: reception to year 11 (age 4 to 16 years old) in England (2023 to 2024 season compared to the 2022 to 2023 season)

Age group	Influenza vaccine uptake, 2023 to 2024 season (%)	Influenza vaccine uptake, 2022 to 2023 season (%)
Reception	55.9	56.7
Year 1	56.4	56.4
Year 2	55.6	57.5
Year 3	55.8	57.3
Year 4	55.1	56.2
Year 5	54.1	55.6
Year 6	53.3	54.2
Year 7	49.1	45.2
Year 8	45.0	40.7
Year 9	42.5	39.6
Year 10	41.6	1.0
Year 11	35.7	1.0
Total primary school-aged	55.1	56.3
Total secondary school-aged (year 7 to year 9)	45.6	41.9
Total secondary school-aged (year 7 to year 11)	42.8	26.7
Total school-aged (reception to year 9)	52.2	51.9
Total school-aged (reception to year 11)	49.9	44.2

Figure 60. Percentage influenza vaccine uptake for children in school years Reception to Year 6 (age 4 to 10 years rising to 11 years old) (primary school age), collected between 1 September 2023 to 31 January 2024, compared to the previous 4 seasons

Overall vaccine uptake for all children of school age reception to year 11 (age 4 to 16 years old) by local authority (not shown here) ranged from 40.0% (491,957 out of 1,229,314) in London to 57.5% (514,133 out of 894,243) in East of England. Uptake by year group and local authority ranged from:

• 25.0% to 94.9% in reception
• 25.9% to 86.5% in year 1
• 24.7% to 91.6% in year 2
• 25.2% to 77.5% in year 3
• 23.2% to 79.5% in year 4
• 23.3% to 74.9% in year 5
• 22.5% to 74.4% in year 6
• 22.3% to 90.6% in year 7
• 17.0% to 79.6% in year 8
• 13.8% to 84.8% in year 9
• 13.2% to 79.4% in year 10
• 10.9% to 65.4% in year 11

Further detail on final influenza vaccine uptake data in all cohorts (GP patients, school-aged children and frontline healthcare workers) in England is publicly available.

Scotland

As previously described, the uptake of seasonal influenza vaccine in children is estimated by PHS throughout the season, using a combination of:

• aggregated data returns submitted by staff in all the territorial NHS boards on a weekly basis for those eligible in the following cohorts: 6 months to 2 years at risk, 2 to 5 years not at school, primary and secondary school pupils
• weekly extracts of data downloaded from the national clinical data store (NCDS) which contains individual-level data relating to the childhood schools’ and nurseries’ programmes for one NHS board

The estimated uptake in preschool children (2 to under 5 years old, not yet in school) was 48.8% – this compares with 56.4% in the 2022 to 2023 season. For primary school children the estimated uptake was 69.0% compared with 74.7% in the 2022 to 2023 season. In secondary school children there was an estimated uptake of 52.9% compared with 61.0% in the 2022 to 2023 season. Note, caution should be taken when comparing uptake percentages between the seasons due to differences in data collection methods.

Wales

In Wales, immunisations for 2 and 3 year olds were delivered through general practices, apart from one health board where the majority of 3 year olds were immunised through nursery school immunisations sessions (uptake in these nursery school sessions was 54.5%). National uptake of influenza vaccine in 2 and 3 year olds decreased slightly in the 2023 to 2024 season. Uptake of influenza vaccine for children aged 2 years was 41.3% (compared to 42.9% in the 2022 to 2023 season ), for 3 year olds it was 44.1% (compared to 44.7% in the 2022 to 2023 season ). For the whole group of children aged 2 and 3 years, uptake was 42.8% (compared to 43.8% in the 2022 to 2023 season).

The childhood influenza programme in Wales includes all primary and secondary school children. Uptake in school children decreased slightly. Primary school children aged 4, 5, 6, 7, 8, 9 and 10 years, received their vaccinations in school immunisation sessions and uptake was 60.8%, 62.7%, 62.3%, 63.2%, 61.9%, 60.9% and 61.3% in each of these groups respectively. For the group as a whole, uptake was 61.9% (compared to 63.8% in the 2022 to 2023 season). Secondary school children aged 11, 12, 13, 14 and 15 years, received their vaccinations in school immunisation sessions and uptake was 55.5%, 51.3%, 49.5%, 47.8% and 44.1% in each of these groups respectively. For the group as a whole, uptake was 49.7% (compared to 54.4% in the 2022 to 2023 season).

Northern Ireland

Caution should be used when considering influenza vaccination uptake rates from 2021 to 2022 in pre-school children in comparison to previous seasons, due to the introduction of the new VMS involving new methods of recording and extracting influenza vaccine data. Influenza vaccinations administered by Trust School Nursing Teams are recorded in the Child Health System (CHS), similar to previous seasons while GP administered vaccinations are recorded in VMS.

In 2023 to 2024 the childhood influenza vaccination programme continued to include all pre-school children aged 2 to 4 years old, all primary school aged children (years 1 to 7) and post primary school children in years 8 to 12. The former group were offered vaccination through primary care, with the latter 2 groups offered vaccination through school health teams. The vaccination uptake rate in 2023 to 2024 for pre-school children aged 2 to 4 years old was 32.9% (compared to 33.0% in 2023 to 2023). The vaccination uptake rate for children in primary school (aged approximately 4 to 11 years old) was 68.6% (compared to 70.7% in 2022 to 2023). In 2021 to 2022, Northern Ireland expanded the vaccination programme from all year 8 children (introduced in 2020 to 2021) to include post-primary school children (years 8 to 12) with an uptake rate of 56.5% in 2023 to 2024. In 2022 to 2023, uptake within this group was 61.0%. These year groups were vaccinated through school clinics and a small number by GP. Uptake figures may be subject to revision with improvements in data collection and reporting.

Vaccine effectiveness

Vaccine effectiveness against acute respiratory infection presenting in primary care

In England, Scotland and Wales for the 2023 to 2024 season, influenza vaccine effectiveness (VE) was estimated using a test-negative study design. VE is presented against influenza in those presenting within primary care with symptoms of acute respiratory infection. Infection data were collected through 3 sentinel GP-based swabbing schemes in England (RCGP), Scotland (CARI) and Wales (as described in the microbiological surveillance section of this report). Vaccination status of study participants was obtained through questionnaire at the time of swabbing (Wales) or data linkage with immunisation databases (England, Scotland) and GP records (England).

In children aged 2 to 17 years, the overall adjusted influenza vaccine effectiveness (VE) in the 2023 to 2024 season was 54% (95% confidence interval (CI): 45% to 61%) against all laboratory confirmed influenza (Figure 61). In adults aged 18 to 64 years, the overall VE was 49% (95% CI: 42% to 56%) against all laboratory confirmed influenza. In adults aged 65 years and over, the overall VE was 46% (95% CI: 29% to 59%).

In subtype-specific analyses across the age groups, moderate VE was demonstrated against influenza A(H1N1) and A(H3N2). Protection against influenza B was good in children and adults aged 18 to 64 years, but it was not possible to estimate VE in adults aged 65 years and over due to low case numbers.

Figure 61. Adjusted vaccine effectiveness against acute respiratory infection presentation in primary care with laboratory-confirmed influenza, by influenza subtype and age group, 2023 to 2024 season, England, Scotland and Wales [note 16][note 17]

[note 16]: Adjusted for week of sample, age group, sex, nation, clinical risk status.

[note 17]: it was not possible to estimate VE against influenza B in adults aged 65 years and over due to low case numbers.

Vaccine effectiveness against hospitalization

England

In England for the 2023 to 2024 season influenza VE was also estimated using a test-negative study design against hospitalization. VE is presented against influenza in those requiring hospitalisation with a diagnosis consistent with acute respiratory infection in England during the period week 40 2023 to week 11 2024. Infection and admission data was collected through the Respiratory DataMart surveillance scheme and Secondary Uses Service (SUS) data. Vaccination status of study participants was obtained through data linkage with the Immunisation Information System (IIS).

In children aged 2 to 17 years, the overall adjusted VE in the 2023 to 2024 season was 54% (95% CI: 42% to 63%) against all laboratory confirmed influenza. In adults aged 18 to 64 years, the overall VE was 31% (95% CI: 21% to 40%) against all laboratory confirmed influenza. In adults aged 65 years and over, the overall VE was 30% (95% CI: 22% to 37%).

Overall VE reflects protection against influenza A, which predominated. However, only a proportion of influenza A positive samples were further subtyped, hence there was greater uncertainty in subtype-specific VE analyses across the age groups. Moderate VE was demonstrated against A(H1N1). Low to moderate VE was demonstrated against influenza A(H3N2), but confidence intervals crossed zero in adults aged 18-64. Protection against influenza B was good in children but uncertain in adults due to low case numbers- in those aged 65 years and above there was insufficient data to estimate vaccine effectiveness (Figure 62).

Figure 62. Adjusted vaccine effectiveness against hospitalisation with a diagnosis consistent with an acute respiratory infection and laboratory-confirmed influenza by influenza subtype and age group, the 2023 to 2024 season, England [note 18][note 19]

[note 18]: It was not possible to estimate VE against influenza B in adults aged 65 years and over due to low case numbers.

[note 19]: Adjusted for week of sample, age group, UKHSA region, clinical risk status.

Scotland

In Scotland, in the 2023 to 2024 season, influenza vaccine effectiveness (VE) was measured using a test-negative study design. VE is presented against influenza hospitalisation among individuals admitted to hospital, as an emergency admission, with a respiratory related diagnosis. This study estimated the odds of testing positive for influenza among vaccinated individuals compared to unvaccinated individuals in a hospitalised population, as a measure of vaccine effectiveness against hospitalisation. The analysis was performed through the data linkage of influenza testing data from the Electronic Communication of Surveillance in Scotland (ECOSS), hospital admission data from Scottish Morbidity Records 01 (SMR01) database and vaccination status of patients from the national clinical data store (NCDS). Study period for the analysis was 3 October 2023 to 29 March 2024.

In children aged 2 to 17 years, the overall adjusted VE in the 2023 to 2024 season was 74% (95% CI 63% to 81%) against hospitalisation with a respiratory condition and any laboratory-confirmed influenzas. In adults aged 18 to 64 years, the overall VE was 48% (95% CI 38% to 55%) and in adults aged 65 years and over, overall VE was 41% (95% CI 33% to 49%). For all influenza types VE was higher in children than older age groups.

In subtype-specific analyses across the age groups, moderate to high VE was demonstrated against influenza A(H3N2) for those aged 2 to 17 years, and aged 65 years and over. High VE was observed against influenza A(H1N1) for those aged 2 to 17 years and 18 to 64 years. High VE was found against influenza B in those aged 2 to 17 years and 18 to 64 years; however, these results should be interpreted with caution due to low circulation of influenza B in the study period resulting in wide confidence intervals. For those aged 65 years and above there was insufficient data to estimate VE against influenza B (Figure 63).

Figure 63. Estimated vaccine effectiveness against hospitalisation with a respiratory condition and laboratory confirmed influenza by subtype and age group, the 2023 to 2024 season, Scotland [note 20][note 21]

[note 20]: It was not possible to estimate VE against influenza B in adults aged 65 years and over due to low case numbers.

[note 21]: Adjusted for time during season, age, sex, deprivation and number of clinical risk groups.

Avian and other zoonotic influenza

Avian influenza A(H5N1)

Since 2003 and up to 24 May 2024, over 890 human cases of avian influenza A(H5N1) have been reported worldwide with a case fatality ratio of over 50%. Since 2020, the avian influenza A(H5N1) clade 2.3.4.4b has become widespread in birds, with some limited spillover to non-avian species and rare detections in humans.

On 25 March 2024, the US reported an outbreak of avian influenza A(H5N1) clade 2.3.4.4b, genotype B3.13 in dairy cattle, and one associated human case. The human case had exposure to cattle presumed to be infected with the virus on a commercial dairy cattle farm in Texas and developed conjunctivitis before recovering.

A second human case was reported on 22 May 2024, in an individual in Michigan who also had exposure to infected dairy cattle. Influenza A infection is exceptionally rare in cows, and avian influenza virus infection of cattle has never been seen. The US outbreak is ongoing, with confirmed A(H5N1) genotype B3.13 detections now reported across 9 states. This specific genotype has not been detected in birds or mammals outside of the US.

In the UK, an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza identified 4 human detections of influenza A(H5N1) clade 2.3.4.4b from 3 geographically distinct sites in 2023. The last human detection in the UK was in July 2023 and on 29 March 2024. The UK self-declared zonal freedom from highly pathogenic avian influenza.

Avian influenza zoonoses summary

In 2021 there was an increase in detections of both influenza A(H5N6) and influenza A(H9N2) in humans, internationally, with sporadic detections continuing to date. Up to 24 May 2024 there have been 91 laboratory-confirmed cases of A(H5N6) in humans, with the most recent case reported from China with onset on 13 April 2024. Up to 24 May 2024 there have been at least 99 laboratory-confirmed cases of A(H9N2) in humans, including 2 deaths, reported since 2015. The latest case was reported from Vietnam with an onset date of 10 March 2024. Most human cases of avian influenza viruses are exposed through contact with infected poultry or contaminated environments, including live poultry markets or domestically kept birds. Since the viruses continue to be detected in animals and environments, further human cases can be expected, however current epidemiological and virological evidence suggests that these viruses have not acquired the ability to undergo sustained transmission among humans. It is important to ensure that imported cases of suspect avian influenza are detected promptly to ensure public health measures including infection control can be rapidly put in place to minimise any risk of onward transmission.

Swine influenza zoonoses summary

On 25 November 2023, UKHSA notified the WHO of a human case of swine-origin influenza A(H1N2)v clade 1B.1.1, detected in England through routine community respiratory virus surveillance. The case fully recovered. While similar viruses have been detected in pigs in England, and the case lived in a region with pig farms, no direct contact between the case and pigs was reported. No other confirmed cases associated with the event were reported.

There continue to be some sporadic reports internationally of human infections with swine influenza viruses such as influenza A(H3N2)v, influenza A(H1N1)v and influenza A(H1N2)v. Close contact with infected pigs or attendance at sites such as farms and agricultural fairs continue to pose an exposure risk.

Middle East respiratory syndrome coronavirus (MERS-CoV) infections

Since the World Health Organization (WHO) first reported cases of Middle East respiratory syndrome coronavirus (MERS-CoV) in April 2012, a total of 2,613 laboratory confirmed cases have been reported globally to WHO up to April 2023. This includes 941 fatal cases (case fatality ratio of 36%). The majority of these cases have occurred in countries in the Arabian Peninsula. A feature of MERS-CoV is its ability to cause large outbreaks within healthcare settings. Local secondary transmission following importation has been reported from several countries including the UK, France, Tunisia, and the Republic of Korea.

A total of 5 cases of MERS-CoV (3 imported and 2 linked cases) have been confirmed in the UK through ongoing surveillance since September 2012.

In April and May 2014, 2 laboratory confirmed cases transited through London Heathrow Airport on separate flights to the US. Contact tracing of flight contacts did not identify any further cases.

Further information on management and guidance of possible cases is available online. The latest ECDC MERS-CoV risk assessment highlights that risk of widespread transmission of MERS-CoV remains very low.

General comments on methodology

The moving epidemic method (MEM) is used by the European Centre for Disease Prevention and Control (ECDC) to standardise reporting of influenza activity across Europe. It has been adopted by the UK and is presented for GP ILI consultation rates for each UK scheme and for the hospitalisation and ICU admissions rate through the SARI Watch scheme. Thresholds of influenza activity intensity using historic data. Prior to the COVID-19 pandemic, the MEM thresholds were calculated using historic data from past 5 seasons. However, due to low circulation of influenza due to measures put in place during the COVID-19 pandemic, seasons 2020 to 2021 and 2021 to 2022 were excluded from the calculations.

Measures put in place during the COVID-19 pandemic affected the transmission of influenza and other respiratory viruses during this period. Although these restrictions and interventions were lifted by the 2022 to 2023 season influenza season, lasting changes in hand hygiene, healthcare-seeking patterns and social distancing measures continue to be important to consider when interpreting the surveillance indicators presented in this report, particularly when comparing to pre-pandemic seasons.

Official statistics designation

Our statistical practice is regulated by the Office for Statistics Regulation (OSR). OSR sets the standards of trustworthiness, quality and value in the Code of Practice for Statistics that all producers of official statistics should adhere to. You are welcome to contact us directly by emailing respdsr.enquiries@ukhsa.gov.uk with any comments about how we meet these standards. Alternatively, you can contact OSR by emailing regulation@statistics.gov.uk or via the OSR website.

The UKHSA is committed to ensuring that these statistics comply with the Code of Practice for Statistics. This means users can have confidence in the people who produce UKHSA statistics because our statistics are robust, reliable and accurate. Our statistics are regularly reviewed to ensure they support the needs of society for information.

UKHSA is currently undertaking a formal review of these statistics, to be completed by the end of summer 2024. Following this review, an implementation plan will be developed in the new year to continue to improve the trustworthiness, quality, and value of these statistics. Key continuous improvements made will be highlighted within these statistics for transparency.

Feedback and acknowledgements

Compiled by the Influenza surveillance section, Immunisation and Vaccine-Preventable Diseases Division, UK Health Security Agency with contributions from:

• Public Health Scotland
• Public Health Wales
• Public Health Agency, Northern Ireland
• Royal College of General Practitioners
• I-Sense Flu team, Department of Computer Science, University College London
• SIREN cohort study team, UK Health Security Agency
• Real-time Syndromic Surveillance team, UK Health Security Agency
• Co- and secondary infections team, UK Health Security Agency
• Respiratory Virus Unit, Colindale, UK Health Security Agency

For queries relating to this document, please contact respdsr.enquiries@ukhsa.gov.uk