Winter Coronavirus (COVID-19) Infection Study: estimates of epidemiological characteristics, 14 March 2024
Updated 11 June 2024
Applies to England and Scotland
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This publication describes the prevalence and incidence rate of the SARS-CoV-2 virus in England and Scotland, and the infection hospitalisation risk of the SARS-CoV-2 virus in England. 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. Information on test positivity is available from the ONS Winter CIS publication.
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. The infection hospitalisation risk (IHR) measures the risk of hospital admission given that an individual has been infected with the SARS-CoV-2 virus.
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.
Main points
Prevalence
Prevalence of SARS-CoV-2 in England and Scotland remained broadly stable over the 2 weeks leading up to 6 March 2024.
In England and Scotland combined, the estimated prevalence of SARS-CoV-2 on 6 March 2024 was 0.7% (95% Credible Interval (CrI): 0.5% to 1.0%) which is equivalent to around 418,000 individuals (95% CrI: 278,000 to 616,000) infected with SARS-CoV-2 in England and Scotland combined. This corresponds to around 1 in 143 (95% CrI: 1 in 200 to 1 in 100) individuals infected with SARS-CoV-2.
In England, the estimated prevalence of SARS-CoV-2 on 6 March 2024 was 0.7% (95% CrI: 0.5% to 1.0%), which is equivalent to around 388,000 individuals (95% CrI: 257,000 to 573,000) infected with SARS-CoV-2 in England. This corresponds to 1 in 143 (95% CrI: 1 in 200 to 1 in 100) individuals infected with SARS-CoV-2.
In Scotland, the estimated prevalence of SARS-CoV-2 on 6 March 2024 was 0.6% (95% CrI: 0.3% to 0.9%), which is equivalent to around 30,000 individuals (95% CrI: 18,000 to 48,000) infected with SARS-CoV-2. This corresponds to around 1 in 167 (95% CrI: 1 in 333 to 1 in 111) individuals infected with SARS-CoV-2.
Prevalence was broadly stable in all age groups over the 2 weeks leading up to 6 March 2024.
Incidence
SARS-CoV-2 incidence was broadly stable in England and Scotland over the 2 weeks leading up to 3 March 2024.
In England and Scotland combined, the estimated incidence rate of SARS-CoV-2 per 100,000 individuals on 3 March 2024 was 73 (95% CrI: 40 to 132) which is equivalent to around 44,000 (95% CrI: 23,800 to 79,100) individuals newly infected with SARS-CoV-2 per day.
In England, the estimated incidence rate of SARS-CoV-2 per 100,000 on 3 March 2024 was 74 (95% CrI: 40 to 134) which is equivalent to around 40,800 (95% CrI: 22,200 to 73,100) individuals newly infected with SARS-CoV-2.
In Scotland, the estimated incidence rate of SARS-CoV-2 per 100,000 on 3 March 2024 was 60 (95% CrI: 31 to 113) which is equivalent to around 3,140 (95% CrI: 1,610 to 5,930) individuals newly infected with SARS-CoV-2 in Scotland.
Incidence was broadly stable in all age groups over the 2 weeks leading up to 3 March 2024.
Infection hospitalisation risk
In England, between 14 November 2023 and 29 February 2024, the risk of an individual being hospitalised with a SARS-CoV-2 infection was 0.45% (95% CrI: 0.42% to 0.47%). This corresponds to a 1 in 224 (95% CrI: 1 in 214 to 1 in 239) chance of those infected being hospitalised.
In England, the infection hospitalisation risk was highest in those aged 75 and over. The risk of an individual aged 75 years and over being hospitalised with a SARS-CoV-2 infection is 3.3% (95% CrI: 2.8% to 3.9%). This corresponds to a 1 in 30 (95% CrI: 1 in 26 to 1 in 36) chance of those infected being hospitalised.
In England, the infection hospitalisation risk was lowest in those aged between 6 to 17 years. The risk of an individual aged between 6 to 17 years being hospitalised with a SARS-CoV-2 infection is 0.025% (95% CrI: 0.020% to 0.032%). This corresponds to a 1 in 3,930 (95% CrI: 1 in 3,080 to 1 in 4,920) chance of those infected being hospitalised.
Figure 1: Estimates of prevalence over time in combined England and Scotland between 14 November 2023 and 6 March 2024.
Estimated Prevalence in England and Scotland
On 6 March 2024, the estimated prevalence of SARS-CoV-2 in England and Scotland was 0.7% (95% CrI: 0.5% to 1.0%) (Figure 1 and Table 1). Prevalence remained broadly stable over the past two weeks from 0.8% (95% CrI: 0.7% to 1.1%) from the 21 February 2024.
On 6 March 2024, the prevalence of SARS-CoV-2 was estimated to be 0.7% (95% CrI: 0.5% to 1.0%) in England and 0.6% (95% CrI: 0.3% to 0.9%) in Scotland. The estimated prevalence in England and Scotland remained broadly stable over the past two weeks from 0.9% (95% CrI: 0.7% to 1.1%) and 0.7% (95% CrI: 0.5% to 0.9%) from the 21 February 2024, respectively (Figure 2a and 2b).
Figure 2a: Estimates of prevalence over time in England between 14 November 2023 and 6 March 2024.
Figure 2b: Estimates of prevalence over time in Scotland between 14 November 2023 and 6 March 2024.
Table 1: The current estimates of prevalence in England and Scotland, and the separate estimates for England and Scotland.
| Date | Location | Prevalence |
|---|---|---|
| 06/03/2024 | England and Scotland | 0.7% (95% CrI: 0.5% to 1.0%) |
| 06/03/2024 | England | 0.7% (95% CrI: 0.5% to 1.0%) |
| 06/03/2024 | Scotland | 0.6% (95% CrI: 0.3% to 0.9%) |
Estimated Prevalence in England and Scotland by Age
The estimated prevalence for the age groups in England and Scotland from the 14 November to 6 March 2024 is referred to in Figure 3 and Table 2. Prevalence remained broadly stable over the last two 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 6 March 2024.
Table 2: The combined England and Scotland current estimates for prevalence by age group.
| Date | Age group | Prevalence |
|---|---|---|
| 06/03/2024 | 3 to 17 years | 1.0% (95% CrI: 0.5% to 1.8%) |
| 06/03/2024 | 18 to 34 years | 0.7% (95% CrI: 0.4% to 1.1%) |
| 06/03/2024 | 35 to 44 years | 0.9% (95% CrI: 0.6% to 1.4%) |
| 06/03/2024 | 45 to 54 years | 0.8% (95% CrI: 0.5% to 1.2%) |
| 06/03/2024 | 55 to 64 years | 0.5% (95% CrI: 0.3% to 0.7%) |
| 06/03/2024 | 65 to 74 years | 0.4% (95% CrI: 0.2% to 0.6%) |
| 06/03/2024 | 75 years and over | 0.3% (95% CrI: 0.2% to 0.5%) |
Estimated Prevalence in the Regions of England
The estimated prevalence for the regions of England from the 14 November to 6 March 2024 is referred to in Figure 4 and Table 3. Prevalence was broadly stable in all regions over the past two 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 6 March 2024.
Table 3: Current estimates of prevalence in the regions of England.
| Date | Region | Prevalence |
|---|---|---|
| 06/03/2024 | North East | 0.8% (95% CrI: 0.5% to 1.3%) |
| 06/03/2024 | North West | 0.6% (95% CrI: 0.4% to 1.0%) |
| 06/03/2024 | Yorkshire and The Humber | 0.7% (95% CrI: 0.4% to 1.0%) |
| 06/03/2024 | East Midlands | 0.8% (95% CrI: 0.5% to 1.2%) |
| 06/03/2024 | West Midlands | 0.7% (95% CrI: 0.5% to 1.2%) |
| 06/03/2024 | East of England | 0.7% (95% CrI: 0.4% to 1.1%) |
| 06/03/2024 | London | 0.7% (95% CrI: 0.4% to 1.0%) |
| 06/03/2024 | South East | 0.7% (95% CrI: 0.5% to 1.1%) |
| 06/03/2024 | South West | 0.8% (95% CrI: 0.5% to 1.2%) |
Estimated Incidence rate in England and Scotland
On 3 March 2024, the estimated incidence rate of SARS-CoV-2 per 100,000 individuals in England and Scotland was 73 (95% CrI: 40 to 132), Figure 5 and Table 4. The incidence rate was broadly stable over the past 2 weeks from 78 (95% CrI: 60 to 101) on 18 February 2024.
On 3 March 2024, the incidence rate of SARS-CoV-2 per 100,000 individuals was estimated to be 74 (95% CrI: 40 to 134) in England and 60 (95% CrI: 31 to 113) in Scotland. The incidence rate in England and Scotland was broadly stable over the past 2 weeks from 80 (95% CrI: 61 to 103) and 64 (95% CrI: 45 to 89) on 18 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 3 March 2024.
Figure 6a: Estimates of the incidence rate over time in England between 14 November 2023 and 3 March 2024.
Figure 6b: Estimates of the incidence rate over time in Scotland between 14 November 2023 and 3 March 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 |
|---|---|---|
| 03/03/2024 | England and Scotland | 73 (95% CrI: 40 to 132) |
| 03/03/2024 | England | 74 (95% CrI: 40 to 134) |
| 03/03/2024 | Scotland | 60 (95% CrI: 31 to 113) |
Estimated Incidence rate in England and Scotland by Age
The estimated incidence rate for the age groups in England and Scotland from the 14 November to 3 March 2024 is referred to in Figure 7 and Table 5. The incidence rate was broadly stable across all age groups over the two weeks leading up to 3 March. 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 3 March 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 |
|---|---|---|
| 03/03/2024 | 3 to 17 years | 112 (95% CrI: 51 to 240) |
| 03/03/2024 | 18 to 34 years | 69 (95% CrI: 31 to 140) |
| 03/03/2024 | 35 to 44 years | 95 (95% CrI: 49 to 177) |
| 03/03/2024 | 45 to 54 years | 83 (95% CrI: 45 to 153) |
| 03/03/2024 | 55 to 64 years | 49 (95% CrI: 25 to 90) |
| 03/03/2024 | 65 to 74 years | 38 (95% CrI: 20 to 70) |
| 03/03/2024 | 75 years and over | 27 (95% CrI: 13 to 52) |
Estimated incidence rate in the Regions of England
The estimated incidence rate for the regions of England from the 14 November to 3 March 2024 is referred to in Figure 8 and Table 6. Incidence was broadly stable 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 3 March 2024.
Table 6: Current estimates of the incidence rate in the regions of England.
| Date | Region | Incidence rate per 100,000 |
|---|---|---|
| 03/03/2024 | North East | 82 (95% CrI: 41 to 170) |
| 03/03/2024 | North West | 65 (95% CrI: 33 to 122) |
| 03/03/2024 | Yorkshire and The Humber | 69 (95% CrI: 35 to 131) |
| 03/03/2024 | East Midlands | 84 (95% CrI: 42 to 160) |
| 03/03/2024 | West Midlands | 78 (95% CrI: 40 to 148) |
| 03/03/2024 | East of England | 74 (95% CrI: 39 to 138) |
| 03/03/2024 | London | 69 (95% CrI: 36 to 129) |
| 03/03/2024 | South East | 75 (95% CrI: 40 to 139) |
| 03/03/2024 | South West | 82 (95% CrI: 43 to 150) |
Infection Hospitalisation Risk
The estimated infection hospitalisation risk (IHR) for England from 14 November 2023 to 29 February is 0.45% (95% CrI: 0.42% to 0.47%). This corresponds to a 1 in 224 (95% CrI: 1 in 214 to 1 in 239) chance of going to hospital with a SARS-CoV-2 infection.
The IHR for each age group is provided in Table 7. The IHR estimate ranges from 0.025% (95% CrI: 0.020% to 0.032%) hospitalisation risk per infection in the 6 to 17 years age group to 3.3% hospitalisation risk per SARS-CoV-2 infection in those aged 75 years and over. This corresponds to a 1 in 3,930 (95% CrI: 1 in 3,080 to 1 in 4,920) chance of going to hospital with a SARS-CoV-2 infection for those aged 6 to 17 years, compared to a 1 in 30 (95% CrI: 1 in 26 to 1 in 36) chance of going to hospital with a SARS-CoV-2 infection in those aged over 75 years.
There are multiple uncertainties that feed into the calculation of an IHR including the incidence itself, the delay distribution from infection to hospitalisation, and hospitalisation definition. Caution should be taken when interpreting, and making comparisons to estimates from other studies. The IHR represents an average risk for the average person in a subgroup at a population level, individual risk will vary due to co-morbidities and other factors. This study has used the definition for SARS-CoV-2 hospitalisations provided by NHS England. This means the IHR is calculated using hospitalisations (admission and diagnosis) of individuals with SARS-CoV-2.
Table 7: For England only, the estimated IHR across age groups.
| Date | Age group | Infection hospitalisation risk |
|---|---|---|
| 14/11/2023 to 29/02/2024 | 6 to 17 years | 0.025% (95% CrI: 0.020% to 0.032%) |
| 14/11/2023 to 29/02/2024 | 18 to 34 years | 0.033% (95% CrI: 0.027% to 0.041%) |
| 14/11/2023 to 29/02/2024 | 35 to 44 years | 0.041% (95% CrI: 0.035% to 0.048%) |
| 14/11/2023 to 29/02/2024 | 45 to 54 years | 0.076% (95% CrI: 0.066% to 0.089%) |
| 14/11/2023 to 29/02/2024 | 55 to 64 years | 0.21% (95% CrI: 0.18% to 0.24%) |
| 14/11/2023 to 29/02/2024 | 65 to 74 years | 0.70% (95% CrI: 0.61% to 0.81%) |
| 14/11/2023 to 29/02/2024 | 75 years and over | 3.3% (95% CrI: 2.8% to 3.9%) |
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.
The infection hospitalisation risk (IHR) is a measure of the risk of hospitalisation given that an individual has been infected with the SARS-CoV-2 virus. The IHR can be estimated indirectly calculated by temporally matching the incidence rate and the number of hospitalisations that occur over time, which adjusts for the delay from an individual becoming infected to going to hospital, and then estimating the proportion of newly infected individuals that subsequently were hospitalised. Daily hospitalisations data come from the NHS England COVID-19 Hospital Statistics. An estimate of the IHR is provided for England only, with an estimate for each age group, and these are combined using re-weighting to provide an overall figure. No data from Scotland were included due to differing definitions of a COVID-19 hospitalisation between the two countries. A separate IHR for Scotland will be provided in a future release.
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. Hospitalisation counts in England are provided by NHS England.
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
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.
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.