Winter Coronavirus (COVID-19) Infection Study: estimates of epidemiological characteristics, 29 February 2024
Updated 14 March 2024
Applies to England and Scotland
© Crown copyright 2024
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This publication describes the prevalence and incidence rate of the SARS-CoV-2 virus in England and Scotland. SARS-CoV-2 is a coronavirus that causes COVID-19.
Test positivity describes the proportion of lateral flow device (LFD) tests taken by participants in the study that are positive for SARS-CoV-2. However, because tests are imperfect, some tests will return false negative results when an infected individual is tested. A very small number of positive tests may also be false positives when an uninfected individual is tested.
Prevalence is an estimate of the proportion of the population who are infected with SARS-CoV-2. It is calculated by adjusting positivity to account for imperfect test performance. This correction is important in this study as LFD tests are used. LFD tests return more false negatives than the polymerase chain reaction (PCR) tests that were used in the previous Office for National Statistics (ONS) Coronavirus (COVID-19) Infection Survey (CIS). Estimates of prevalence are useful for individuals who want to understand their risk of exposure to SARS-CoV-2. The incidence rate is an estimate of how many new infections of SARS-CoV-2 occur each day per 100,000 people in the population.
To ensure estimates are representative of the wider population, the prevalence and incidence estimates from this cohort are adjusted through reweighting. The reweighting approach used by this study aims to provide an estimate of prevalence and incidence that are representative of the whole population in terms of age, geography, and sex. The study only includes participants aged over 2 years and therefore all the analyses conducted, and population estimates used in this report do not include individuals aged under 3 years.
In future publications we will include the infection hospitalisation risk, which measures the risk of hospital admission given that an individual has been infected with the SARS-CoV-2 virus. Information on unweighted positivity is available from the ONS Winter CIS publication.
Main points
Prevalence
Prevalence of SARS-CoV-2 in England and Scotland declined over the 2 weeks leading up to 21 February 2024.
In England and Scotland combined, the estimated prevalence of SARS-CoV-2 on 21 February 2024 was 0.9% (95% Credible Interval (CrI): 0.6%, 1.2%) which is equivalent to around 525,000 individuals (95% CrI: 381,000 to 717,000) infected with SARS-CoV-2. This corresponds to around 1 in 111 (95% CrI: 1 in 167 to 1 in 83).
In England, the estimated prevalence of SARS-CoV-2 on 21 February 2024 was 0.9% (95% CrI: 0.6%, 1.2%), which is equivalent to around 489,000 individuals (95% CrI: 355,000 to 667,000) infected with SARS-CoV-2. This corresponds to 1 in 111 (95% CrI: 1 in 167 to 1 in 83).
In Scotland, the estimated prevalence of SARS-CoV-2 on 21 February 2024 was 0.7% (95% CrI: 0.5%, 1.0%), which is equivalent to around 36,000 individuals (95% CrI: 24,000 to 52,000) infected with SARS-CoV-2. This corresponds to around 1 in 143 (95% CrI: 1 in 200 to 1 in 100).
Prevalence declined in those aged 35 years and over in the 2 weeks leading up to 21 February 2024.
Incidence
There is strong evidence for a decline in the incidence of SARS-CoV-2 in England and Scotland over the 2 weeks leading up to 18 February 2024.
In England and Scotland combined, the estimated incidence rate of SARS-CoV-2 per 100,000 individuals on 18 February 2024 was 74 (95% CrI: 45 to 116) which is equivalent to around 44,100 (95% CrI: 27,200 to 69,400) individuals newly infected with SARS-CoV-2 per day.
In England, the estimated incidence rate of SARS-CoV-2 on 18 February 2024 was 75 (95% CrI: 46 to 118) which is equivalent to around 41,100 (95% CrI: 25,300 to 64,400) individuals newly infected with SARS-CoV-2.
In Scotland, the estimated incidence rate of SARS-CoV-2 on 18 February 2024 was 57 (95% CrI: 33 to 95) which is equivalent to around 3,020 (95% CrI: 1,720 to 5,000) individuals newly infected with SARS-CoV-2.
Incidence declined in those aged 45 years and over in the 2 weeks leading up to 18 February 2024.
Figure 1. Estimates of prevalence over time in combined England and Scotland between 14 November 2023 and 21 February 2024
Estimated prevalence in England and Scotland
On 21 February 2024, the estimated prevalence of SARS-CoV-2 in England and Scotland was 0.9% (95% CrI: 0.6%, 1.2%) (Figure 1 and Table 1). Prevalence declined over the past 2 weeks from 1.6% (95% CrI: 1.3%, 2.0%) on 7 February 2024.
On 21 February 2024, the prevalence of SARS-CoV-2 was estimated to be 0.9% (95% CrI: 0.6%, 1.2%) in England and 0.7% (95% CrI: 0.5%, 1.0%) in Scotland. The estimated prevalence in England and Scotland declined over the past 2 weeks from 1.7% (95% CrI: 1.4%, 2.1%) and 1.3% (95% CrI: 1.0%, 1.7%) on 24 January 2024, respectively (Figure 2a and 2b).
Figure 2a. Estimates of prevalence over time in England between 14 November 2023 and 21 February 2024
Figure 2b. Estimates of prevalence over time in Scotland between 14 November 2023 and 21 February 2024
Table 1. The current estimates of prevalence in England and Scotland, and the separate estimates for England and Scotland
| Date | Location | Prevalence |
|---|---|---|
| 21 February 2024 | England and Scotland | 0.9% (95% CrI: 0.6%, 1.2%) |
| 21 February 2024 | England | 0.9% (95% CrI: 0.6%, 1.2%) |
| 21 February 2024 | Scotland | 0.7% (95% CrI: 0.5%, 1.0%) |
Estimated prevalence in England and Scotland by age
The estimated prevalence for the age groups in England and Scotland from 14 November to 21 February 2024 is referred to in Figure 3 and Table 2. Prevalence declined in those aged 35 years and over, remained broadly stable in those aged between 3 and 17 years, and there is strong evidence for a decline in those aged 18 to 34 years and over the last 2 weeks. As the sample sizes used to estimate prevalence for each age group are smaller than the overall study’s sample size, there is higher uncertainty compared to national estimates, which is reflected in wider credible intervals.
Figure 3. Estimates of prevalence over time by age group in combined England and Scotland between 14 November 2023 and 21 February 2024
Table 2. The combined England and Scotland current estimates for prevalence by age group
| Date | Age group | Prevalence |
|---|---|---|
| 21 February 2024 | 3 to 17 years | 1.2% (95% CrI: 0.7%, 2.1%) |
| 21 February 2024 | 18 to 34 years | 0.9% (95% CrI: 0.6%, 1.4%) |
| 21 February 2024 | 35 to 44 years | 1.1% (95% CrI: 0.8%, 1.6%) |
| 21 February 2024 | 45 to 54 years | 0.9% (95% CrI: 0.6%, 1.2%) |
| 21 February 2024 | 55 to 64 years | 0.6% (95% CrI: 0.4%, 0.8%) |
| 21 February 2024 | 65 to 74 years | 0.5% (95% CrI: 0.3%, 0.6%) |
| 21 February 2024 | 75 years and over | 0.4% (95% CrI: 0.3%, 0.6%) |
The estimated prevalence for the regions of England from 14 November to 21 February 2024 is referred to in Figure 4 and Table 3. There was strong evidence for a decline in prevalence in the North East, and East Midlands, and prevalence declined in all other regions over the past 2 weeks.
There is considerable uncertainty in the estimated prevalence for the regions of England. As the sample sizes used to estimate prevalence for each region of England are smaller than the overall study’s sample size, there is higher uncertainty compared to national estimates, which is reflected in wider credible intervals.
Figure 4. Estimates of prevalence over time in regions of England between 14 November 2023 and 21 February 2024
Table 3. Current estimates of prevalence in the regions of England
| Date | Region | Prevalence |
|---|---|---|
| 21 February 2024 | North East | 0.9% (95% CrI: 0.6%, 1.3%) |
| 21 February 2024 | North West | 0.8% (95% CrI: 0.5%, 1.1%) |
| 21 February 2024 | Yorkshire and Humber | 0.8% (95% CrI: 0.6%, 1.2%) |
| 21 February 2024 | East Midlands | 1.0% (95% CrI: 0.7%, 1.4%) |
| 21 February 2024 | West Midlands | 0.9% (95% CrI: 0.6%, 1.3%) |
| 21 February 2024 | East of England | 0.9% (95% CrI: 0.6%, 1.3%) |
| 21 February 2024 | London | 0.8% (95% CrI: 0.6%, 1.2%) |
| 21 February 2024 | South East | 0.9% (95% CrI: 0.7%, 1.3%) |
| 21 February 2024 | South West | 1.0% (95% CrI: 0.7%, 1.5%) |
Estimated incidence rate in England and Scotland
On 18 February 2024, the estimated incidence rate of SARS-CoV-2 per 100,000 individuals in England and Scotland was 74 (95% CrI: 45 to 116), referred to in Figure 5 and Table 4. There is strong evidence for a decline in the incidence rate over the past 2 weeks from 144 (95% CrI: 114 to 182) on 4 February 2024.
On 18 February 2024, the incidence rate of SARS-CoV-2 per 100,000 individuals was estimated to be 75 (95% CrI: 46 to 118) in England and 57 (95% CrI: 33 to 95) in Scotland. There is strong evidence for a decline in the incidence rate in England and Scotland over the past 2 weeks from 144 (95% CrI: 114 to 182) and 147 (95% CrI: 116 to 185) on 4 February 2024, respectively (Figure 6a and 6b).
Figure 5. Estimates of the incidence rate over time in combined England and Scotland between 14 November 2023 and 18 February 2024
Figure 6a. Estimates of the incidence rate over time in England between 14 November 2023 and 18 February 2024
Figure 6b. Estimates of the incidence rate over time in Scotland between 14 November 2023 and 18 February 2024
Table 4: The current estimates of the incidence rate in England and Scotland, and the separate estimates for England and Scotland
| Date | Location | Incidence rate per 100,000 |
|---|---|---|
| 18 February 2024 | England and Scotland | 74 (95% CrI: 45 to 116) |
| 18 February 2024 | England | 75 (95% CrI: 46 to 118) |
| 18 February 2024 | Scotland | 57 (95% CrI: 33 to 95) |
Estimated incidence rate in England and Scotland by age
The estimated incidence rate for the age groups in England and Scotland from 14 November to 18 February 2024 is referred to in Figure 7 and Table 5. The incidence rate declined in those aged 45 years and over, there was strong evidence for a decline in the incidence rate in those aged between 18 and 44 years, and the incidence rate remained broadly stable in those aged between 3 and 17 years over the 2 weeks leading up to 18 February (Figure 3 and Table 2). As the sample sizes used to estimate incidence for each age group are smaller than the overall study’s sample size, there is higher uncertainty compared to national estimates, which is reflected in wider credible intervals.
Figure 7. Estimates of the incidence rate over time by age group in combined England and Scotland between 14 November 2023 and 18 February 2024
Table 5: The combined England and Scotland current estimates for the incidence rate by age group
| Date | Age group | Incidence rate per 100,000 |
|---|---|---|
| 18 February 2024 | 3 to 17 years | 115 (95% CrI: 56 to 228) |
| 18 February 2024 | 18 to 34 years | 76 (95% CrI: 41 to 134) |
| 18 February 2024 | 35 to 44 years | 91 (95% CrI: 54 to 146) |
| 18 February 2024 | 45 to 54 years | 77 (95% CrI: 47 to 122) |
| 18 February 2024 | 55 to 64 years | 45 (95% CrI: 27 to 72) |
| 18 February 2024 | 65 to 74 years | 35 (95% CrI: 21 to 55) |
| 18 February 2024 | 75 years and over | 28 (95% CrI: 16 to 46) |
Estimated incidence rate in the regions of England
The estimated incidence rate for the regions of England from 14 November to 18 February 2024 is referred to in Figure 8 and Table 6. There is strong evidence for a decline in all regions over the past 2 weeks. There is considerable uncertainty in the estimated incidence rate for the regions of England. As the sample sizes used to estimate incidence rate for each region of England are smaller than the overall study’s sample size, there is higher uncertainty compared to national estimates, which is reflected in wider credible intervals.
Figure 8. Estimates of the incidence rate over time in regions of England between 14 November 2023 and 18 February 2024
Table 6: Current estimates of the incidence rate in the regions of England
| Date | Region | Incidence rate per 100,000 |
|---|---|---|
| 18 February 2024 | North East | 73 (95% CrI: 41 to 127) |
| 18 February 2024 | North West | 66 (95% CrI: 38 to 108) |
| 18 February 2024 | Yorkshire and Humber | 72 (95% CrI: 41 to 119) |
| 18 February 2024 | East Midlands | 83 (95% CrI: 49 to 137) |
| 18 February 2024 | West Midlands | 76 (95% CrI: 43 to 128) |
| 18 February 2024 | East of England | 75 (95% CrI: 44 to 122) |
| 18 February 2024 | London | 67 (95% CrI: 39 to 111) |
| 18 February 2024 | South East | 79 (95% CrI: 47 to 129) |
| 18 February 2024 | South West | 86 (95% CrI: 52 to 141) |
Methodology
Demographics are over or under-represented in the survey sample, and it is important to account for this to produce estimates of SARS-CoV-2 prevalence and incidence that are representative of population.
A Bayesian Multilevel Regression and Post-stratification (MRP) approach is used to estimate the incidence rate and prevalence for different subgroups (geography, age, and sex). MRP helps to reduce the uncertainty in subgroups estimates that might be under-represented or under-sampled in the original survey. The model partially pools information across subgroups to improve the precision and accuracy of estimates in under-represented groups. Without this approach to pooling information, estimates may not accurately reflect temporal trends in SARS-CoV-2 transmission in the wider population. These model estimates are then re-weighted using the true population size of different subgroups to give a more representative estimate for the target population.
In the survey, once a participant tests positive for SARS-CoV-2, they are asked to take repeat tests every other day until they return 2 negative tests. This repeat testing data is used to estimate the false negative rates of LFDs, over time, for the cohort. This allows us to further model the test sensitivity as it evolves over the epidemic phases. As the study gathers more data the diagnostic performance of the LFD tests will be updated. The repeat testing data is also used to estimate the duration of positivity of SARS-CoV-2 infections, which is further used to calculate incidence and prevalence.
Positivity must be adjusted for the imperfect test sensitivity of LFD tests (causing falsely negative test results) to estimate of prevalence. False negatives occur when an individual is truly infected with SARS-CoV-2 but receives a negative test result. The model estimates the false negative rate we expect to observe in the data, allowing us to adjust for the presence of false negatives when calculating incidence. The estimated false negative rate varies over time, depending on epidemic behaviour, and across different age groups. The model also adjusts for test specificity, however due to the high specificity of LFD tests (very small chance of a falsely positive test), it has a minimal impact on the estimated incidence.
Incidence is calculated by estimating the rate at which new infections of SARS-CoV-2 occur within the subgroups analysed over time, reported as a rate per 100,000 people. Prevalence is a measure of the proportion of the population infected with SARS-CoV-2. An infected individual’s exposure date to SARS-CoV-2 occurs prior to testing positive. Incidence is a measure of new infections by exposure date and therefore, it is reported with a temporal delay relative to prevalence. The prevalence at a given point in time can be expressed in terms of the number of recently infected individuals that have not yet cleared the virus. Using the repeat testing data collected in the survey, we estimate how long someone is likely to test positive for, known as the duration of positivity. The model uses the duration of positivity, obtained from follow up testing, to estimate a time series of incidence rates, by demographic group, that is most credible in generating the observed positivity in the survey cohort. This allows the model to then estimate the prevalence at each point in time from the temporal pattern of incidence rates.
Individuals appear to be more likely to test earlier or before the testing window if they are symptomatic. As a result, tests taken earlier in the window are more likely to be positive than tests taken later in the testing window. Additionally, our model incorporates an adjustment for this effect, based upon which day of their testing window an individual took their test. This testing behaviour pattern changed over the winter bank holiday period which has been accounted for in the model.
Note, all the analyses conducted in this report do not include individuals aged under 3 years.
Data sources
Based on responses from the Winter Coronavirus (COVID-19) Infection Study (Winter CIS), commissioned and funded by UK Health Security Agency (UKHSA), to deliver real-time information to help assess the effects of COVID-19 on the lives of individuals and the community, and help understand the potential winter pressures on our health services. The study has been launched jointly by ONS and UKHSA, with data collected via online questionnaire completion and self-reported lateral flow device (LFD) results from previous participants of the COVID-19 Infection Survey (CIS). The ONS 2023 to 2024 population projections will be published alongside the report on positivity.
Authors
Alex Glaser – UKHSA
Alexander Phillips – UKHSA, University of Liverpool
Andre Charlett – UKHSA
Christopher Overton – UKHSA, University of Liverpool
Jonathon Mellor – UKHSA
Julie Day – UKHSA
Martyn Fyles – UKHSA
Owen Jones – UKHSA
Robert Paton – UKHSA
Steven Riley – UKHSA, Imperial College London
Thomas Ward – UKHSA
Glossary
Prevalence
The estimated proportion of individuals who are infected with the SARS-CoV-2 virus at a given point in time.
Incidence rate
An estimate of how many new infections of SARS-CoV-2 occur each day per 100,000 people in the population.
Infection hospitalisation risk (IHR)
Measures the risk of hospital admission given that an individual has been infected with the SARS-CoV-2 virus.
Infection fatality risk (IFR)
Measures the risk of death given that an individual has been infected with the SARS-CoV-2 virus.
Vaccine effectiveness
How effectively vaccinations protect people from health outcomes such as infection, symptomatic disease, hospitalisation, and mortality.