Official Statistics

National norovirus and rotavirus report, week 44 report: data up to week 42 (22 October 2023)

Updated 11 April 2024

Applies to England

This weekly report covers the 2-week period between 9 to 22 October 2023. Data reported here provide a summary of norovirus and rotavirus activity (including enteric virus outbreaks) in England up to reporting week 42 of the 2023/2024 season.

In this report, the ‘5-season average’ is calculated from the same period during the 5 seasons of 2015/2016, 2016/2017, 2017/2018, 2018/2019 and 2022/2023. Refer to the data sources and reporting caveats for more information and for guidance on interpretation of the trends.

The main messages of this report are:

1. Norovirus laboratory reports have remained lower than the 5-season average in recent weeks and during the 2-week period of the 2023/2024 season (weeks 41 and 42) were 20% less than the 5-season average of the same period.
2. Rotavirus activity during weeks 41 and 42 was comparable with the 5-season average.
3. Overall, the total number of reported Enteric Virus (EV) outbreaks reported during weeks 41 and 42 remained lower than the 5-season average for the same 2-week period.
4. The number of norovirus outbreaks reported to the Hospital Norovirus Outbreak Reporting System (HNORS) in recent weeks was lower than the 5-season average.

Background

No single surveillance system fully captures national changes in norovirus or rotavirus activity; therefore, this report presents data from 4 systems which collectively describe recent trends. Data is reported by season rather than calendar year, in order to capture the winter peak of activity in one reporting period. Refer to the data sources and reporting caveats for more information and for guidance on interpretation of the trends.

Norovirus activity and the timing of the peak in reporting can vary considerably and differ from one season to the next. Transmission is mostly via contact with an infected person, but norovirus can also be spread by consumption of food contaminated with the virus or contact with contaminated surfaces. Norovirus transmission is influenced by many factors including, but not limited to, whether the person has been infected with norovirus recently, community contact patterns and the time of the year. Infections peak in the colder months and therefore norovirus is most prevalent during winter.

Since the 2019/2020 season, norovirus activity in England has been more variable compared with historical trends, likely because of the multiple impacts of the COVID-19 pandemic. Refer to the data sources and reporting caveats for more information on the impact of the pandemic. Throughout the 2019/2020 and 2020/21 seasons, a reduction in reporting to national surveillance was observed. This was followed by unusual activity in the 2021/2022 season, such as a greater proportion of outbreaks reported in educational settings than before the pandemic. Norovirus activity returned to pre-pandemic levels in the 2022/2023 season. In early 2023, norovirus reporting peaked at a level twice as high as that experienced in the decade before the emergence of COVID-19.

Following the introduction of the rotavirus vaccine in July 2013, the total number of laboratory-confirmed rotavirus infections each season has remained low compared with the pre-vaccine period. A 77% decrease in laboratory-confirmed rotavirus infections in infants was observed in the first season following vaccine introduction (1).

UK Health Security Agency (UKHSA) routinely undertakes norovirus characterisation as part of national surveillance to monitor the diversity of circulating strains. This molecular surveillance enables detection of novel strains or emergence of existing strains that could lead to a strain replacement event and which have previously been associated with a temporal shift in norovirus activity (2). Norovirus activity varies from season to season and therefore differences will be observed between every season.

The most commonly detected norovirus genotype worldwide is genogroup II- genotype 4 (GII.4). Historically, between 1995 and 2013 there have been 5 global GII.4 strain replacements events (3, 4). Since the winter of the 2012 to 2013 season and before the emergence of COVID-19 in England, the most frequently detected strain was Norovirus/GII.4/Sydney/2012 or GII.4 Sydney2012-like variants (5).

To enable effective molecular surveillance, it is crucial that samples are obtained from suspected norovirus cases or outbreaks for laboratory confirmation and then norovirus-positive samples are referred on to the enteric virus unit (EVU) for characterisation.

Laboratory surveillance

Data presented here is derived from the Second-Generation Surveillance System (SGSS). Refer to the data sources and reporting caveats for more information and for guidance on interpretation of trends.

The cumulative number of positive norovirus laboratory reports in England (1,125 laboratory reports) up to week 42 of the 2023/2024 season was 25% higher than the 5-season average for the same period (901 laboratory reports). This is mostly because of higher than usual activity at the beginning of the season between weeks 27 to 34.

However, norovirus activity over the 2-week period of weeks 41 and 42, 2023 (133 laboratory reports), was 20% lower than the 5-season average (167 laboratory reports) for the same 2-week period (Figure 1).

Figure 1. Norovirus laboratory reports in England by week during the 2023/2024 season, compared with 5-season average

The cumulative number of positive rotavirus laboratory reports in England up to week 42 of the current season (757 laboratory reports) was 30% higher than the 5-season average for the same period (584 laboratory reports, as seen in Figure 2).

Rotavirus activity has remained stable in recent weeks, the total number of laboratory reports during the 2-week period of weeks 41 and 42, 2023 (52 laboratory reports) was comparable with the 5-season average (53 laboratory reports) for the same period.

Figure 2. Rotavirus laboratory reports in England by week during the 2023/2024 seasons, compared with 5-season average

Outbreak surveillance

Data presented here is derived from HPZone and the Hospital Norovirus Outbreak Reporting System (HNORS). Refer to the data sources and reporting caveats for more information and for guidance on interpretation of the trends.

The cumulative number of EV gastroenteritis outbreaks reported to HPZone up to week 42 of the 2023/2024 season was 29% lower than the 5-season average, at 539 and 757 outbreaks, respectively (Figure 3).

The total number of reported EV outbreaks in weeks 41 and 42 remained lower than the 5-season average for the same 2-week period (43% lower, 79 versus 138, respectively).

The majority of reported EV outbreaks (all suspected or confirmed as norovirus) during the 2-week period of weeks 41 and 42, 2023, occurred in care home settings (66%, as seen in Figure 3). The number of outbreaks reported in educational settings dropped during this 2-week period, coinciding with the October half-term school holidays.

During the 5 seasons from which the 5-season average was calculated, overall, 63% of all reported outbreaks attributed to EVs (norovirus, rotavirus, sapovirus and astrovirus), occurred in care home settings, 20% in educational settings, 11% in hospital settings and 6% in ‘other’ settings. Of the outbreaks attributed to EVs, 99% were reported as suspected and confirmed norovirus outbreaks. Only 14% of reported EV outbreaks were laboratory confirmed as norovirus during the previous 5 seasons.

Figure 3. Enteric virus gastroenteritis outbreaks reported to HPZone in England by setting during the 2023/2024 season, compared with the 5-season average

12 outbreaks have been reported to HNORS up to week 42 of the 2023/2024 season (Figure 4), which is 65% lower than the 5-season average for the same period (35 outbreaks). Overall, 92% of outbreaks were laboratory confirmed as norovirus.

Overall, 80% of outbreaks reported to HNORS were laboratory confirmed as norovirus during the 5 seasons from which the 5-season average was calculated.

Figure 4. Suspected and confirmed norovirus outbreaks reported to HNORS in England by week of occurrence during the 2023/2024 seasons compared with the 5-season average

Molecular surveillance for norovirus

Data presented here is provided by UKHSA’s enteric virus unit (EVU) and is used by UKHSA to monitor circulating norovirus variants. Refer to the data sources and reporting caveats for more information and for guidance on interpretation of trends.

Of the 123 norovirus positive samples characterised during the 2023/ 2024 season to date, 62% (76 out of 123) were genogroup 2 (GII); 37% (46 out of 123) were genogroup 1 (GI) and 1% were mixed (1 out of 123). The most frequent norovirus GII genotypes identified were GII.4 (17%) and GII.3 (12%), the most frequently identified norovirus GI genotypes were GI.3 (15%) and GI.6 (13%). The most commonly identified GII.4 norovirus strain so far this season is Norovirus/GII.4/Sydney/2012-like variants.

Because of low numbers of samples submitted for molecular surveillance in the early part of the season, this data should be interpreted with caution.

Data sources

The Second-Generation Surveillance System (SGSS) is the national laboratory reporting system, recording positive laboratory reports of norovirus and rotavirus.

The Hospital Norovirus Outbreak Reporting System (HNORS) is a web-based scheme for reporting suspected and confirmed norovirus outbreaks in Acute NHS Trust hospitals, and captures information on the disruptive impact these outbreaks have in hospital settings.

HPZone is a web-based case and outbreak management system used by health protection teams (HPTs) to record outbreaks they are notified of and investigate. In England, suspected and confirmed EV outbreaks (norovirus, rotavirus, astrovirus and sapovirus) are reported as ‘Gastroenteritis’ outbreaks. Please note this was not available for this time period.

Norovirus characterisation data is produced by the EVU and is used to monitor the diversity of circulating strains of norovirus in England.

Reporting caveats

Interpretation of trends

In order to capture the winter peak of activity in the reporting period the norovirus and rotavirus season runs from week 27 in year one to week 26 in year 2, that is, week 27 2023 to week 26 2024, July to June. The 2023/2024 season is compared with the 5-season average calculated from the 5-season period of 2015/2016 to 2018/2019 and 2022/2023. The 2019/2020, 2020/2021 and 2021/22 seasons are not included in this calculation. This is because of the adverse impact of the emergence of COVID-19 on surveillance part way through the 2019/2020 season and the continued impact into the 2020/2021 and 2021/2022 seasons. In years with a week 53 (2015 and 2020), data is combined with week 52 data to avoid distortion of the figure.

Under-ascertainment is a recognised challenge in enteric virus surveillance with sampling, testing and reporting criteria known to vary by region. In addition, samples for microbiological confirmation are collected in a small proportion of community outbreaks. Therefore, this report provides an overview of enteric virus activity across England and data should be interpreted with caution.

All surveillance data included in this report is extracted from live reporting systems, is subject to a reporting delay, and the number reported in the most recent weeks may rise further as more reports are received. Therefore, data pertaining to the most recent 2 weeks is not included.

Impact of COVID-19 pandemic on surveillance

UKHSA relaunched the Official Statistics National Norovirus and Rotavirus Report after it was temporarily suspended due to quality issues with the data from the 4 aforementioned data sources during the COVID-19 pandemic period. Additional analyses of this data were undertaken and demonstrated the quality of this data was comparable with the data collected before the pandemic and therefore reporting was allowed to resume as an Official Statistic. Between December 2020 and October 2022 the report was replaced by the National norovirus and rotavirus bulletin to ensure an overview of norovirus and rotavirus activity in England continued to be available to the public (data covering the periods 2020/2021 and 2021/2022 is available at National norovirus and rotavirus bulletins 2020 to 2021: management information and National norovirus and rotavirus bulletins 2021 to 2022: management information).

The COVID-19 pandemic impacted activity across many gastrointestinal pathogen surveillance indicators for England in 2020 and 2021, and reduced norovirus reporting continued into early 2022. The reasons for the reduction in norovirus reporting are considered to be multifactorial. It is likely that the interventions implemented to control COVID-19 led to a reduction in enteric virus transmission. However, when considering the surveillance data reported here, the magnitude of the reduction is unlikely to be wholly attributable to these control measures alone. It is likely that other factors such as, but not limited to, changes in ascertainment, access to health care services and capacity for testing also contributed to the observed reduction because of changes in ascertainment and varied over time. The reduction in norovirus reporting to national surveillance during the 2019/2020 and 2020/2021 seasons also lead to a period of low referral of norovirus-positive samples for characterisation. Therefore, trends for the 2019/2020, 2020/2021 and 2021/2022 seasons should be interpreted with caution.

SGSS data

SGSS data is England only, week number is calculated from specimen date and location is based on laboratory geography. Norovirus data include faecal and lower gastrointestinal tract specimen types only. Reporting may be subject to differences in regional ascertainment.

HPZone data

HPZone data utilises week of date of outbreak entry on to HPZone for analyses due to mandatory completion of the field. While this usually reflects the date of notification, batch reporting of outbreaks can occur.

HNORS data

HNORS reporting is voluntary and variations may reflect differences in ascertainment or reporting criteria by region. National guidance recommends closure of the smallest possible unit in hospitals. Therefore, not all outbreaks reported to HNORS result in whole ward closure (some closures are restricted to bays only) and not all suspected cases are tested. Additionally, not all suspected cases are tested for norovirus, often only a proportion of individuals will be tested in any suspected outbreak.

Week number is calculated from date of first case onset for HNORS data.

From May to October 2019 and during February 2020 the HNORS website was temporarily offline. The reliance on manual data collation during this period may have negatively impacted ascertainment so trends should be interpreted with caution.

Norovirus characterisation data

Norovirus genotype and GII.4 strain characterisation data from the reference laboratory are subject to a reporting delay, and the numbers reported in any week may rise further additional characterisation data become available.

References

1. Atchison and others. ‘Rapid declines in age group–specific rotavirus infection and acute gastroenteritis among vaccinated and unvaccinated individuals within 1 year of rotavirus vaccine introduction in England and Wales’, The Journal of Infectious Diseases: volume 213, pages 243 to 249 (viewed on 17 October 2022)

2. Allen and others. ‘Emergence of the GII-4 norovirus Sydney2012 strain in England, winter 2012–2013’, The Public Library of Science One, volume 2, article: e88978 (viewed on 17 October 2022)

3. Allen and others. ‘Characterisation of a GII-4 norovirus variant-specific surface-exposed site involved in antibody binding’ Virology Journal, volume 6, article number: 150 (viewed on 17 October 2022)

4. Zakikhany and others. ‘Molecular evolution of GII-4 Norovirus strains’ The Public Library of Science One, volume 7, article: e41625 (viewed on 17 October 2022)

5. Ruis C and others. ‘The emerging GII.P16-GII.4 Sydney 2012 norovirus lineage is circulating worldwide, arose by late-2014 and contains polymerase changes that may increase virus transmission’ The Public Library of Science One, volume 6, article: e0179572 (viewed on 17 October 2022)

Further information

Further information about norovirus surveillance and rotavirus surveillance is available on GOV.UK.

Acknowledgements

We are grateful to all who provided data used in this report, including NHS Infection Control and Prevention staff (HNORS users), UKHSA local (HPTs) and UKHSA regional teams (Field Services) and UKHSA Regional Public Health and Collaborating Laboratories.

This report was produced by the Gastrointestinal Infections & Food Safety (One Health) Division, UKHSA.

Please direct any queries or comments to NoroOBK@ukhsa.gov.uk