Coronavirus (COVID-19) Infection Survey, antibody and vaccination data, UK: 10 November 2021

• In England, it is estimated that 93.1% of the adult population (95% credible interval: 91.9% to 94.1%) would have tested positive for antibodies against SARS-CoV-2, the specific virus that causes coronavirus (COVID-19), on a blood test in the week beginning 18 October 2021, suggesting they had the infection in the past or have been vaccinated.

• In Wales, it is estimated that 91.7% of the adult population (95% credible interval: 90.1% to 93.1%) would have tested positive for antibodies against SARS-CoV-2 on a blood test in the week beginning 18 October 2021, suggesting they had the infection in the past or have been vaccinated.

• In Northern Ireland, it is estimated that 90.9% of the adult population (95% credible interval: 86.7% to 93.2%) would have tested positive for antibodies against SARS-CoV-2 on a blood test in the week beginning 18 October 2021, suggesting they had the infection in the past or have been vaccinated.

• In Scotland, it is estimated that 92.5% of the adult population (95% credible interval: 91.0% to 93.7%) would have tested positive for antibodies against SARS-CoV-2 on a blood test in the week beginning 18 October 2021, suggesting they had the infection in the past or have been vaccinated.

About this bulletin

This bulletin presents analysis on past infection and/or vaccination – which we define as testing positive for antibodies to SARS-CoV-2 – for England, Wales, Northern Ireland and Scotland, based on findings from the Coronavirus (COVID-19) Infection Survey (CIS) in the UK. For context, we include estimates from our survey on the percentage of people who reported they have received at least one or more doses of a vaccine against SARS-CoV-2, as well as those who report to having received two or more doses of a COVID-19 vaccine (including booster doses).

In this bulletin, we refer to the following:

Antibodies

We measure the presence of antibodies in people who live in private households to understand who has had coronavirus (COVID-19) in the past, and the impact of vaccination. It takes between two and three weeks after infection or vaccination for the body to make enough antibodies to fight the infection. Antibodies can help prevent individuals from getting the same infection again. If they do get infected, people with antibodies are less likely to have severe symptoms. Once infected or vaccinated, antibodies remain in the blood at low levels and can decline over time. The length of time antibodies remain at detectable levels in the blood is not fully known.

SARS-CoV-2

This is the scientific name given to the specific virus that causes COVID-19.

Data in this bulletin

The analysis on antibodies in this bulletin is based on blood test results taken from a randomly selected subsample of individuals aged 16 years and over who live in private households. The survey excludes those in hospitals, care homes and/or other communal establishments. The blood samples are used to test for antibodies against SARS-CoV-2.

We also present data on the percentage of people aged 16 years and over who report they have received one or more doses of a COVID-19 vaccine since 14 December 2020, and the percentage of people aged 16 years and over who report they have received two or more doses of a COVID-19 vaccine since 15 February 2021.

Our antibodies and vaccination estimates are based on modelling of the people visited in the Coronavirus (COVID-19) Infection Survey (CIS). Further information on our method to model antibodies and vaccinations can be found in our methods article.

We produce weekly modelled estimates using standard calendar weeks starting Monday. To provide the most timely and accurate estimates possible for antibody positivity, the model will include data for the first four to seven days of the most recent week available, depending on the availability of test results. The antibody and vaccination estimates for the most recent week in this publication includes data from 18 October to 24 October 2021.

Antibodies data are currently unavailable for the period from 27 August to 5 September 2021 and there are a lower number of test results available from 6 September to 12 September 2021 because of a temporary issue in the laboratory testing process. We are working with our laboratories to resolve this issue and will receive these data in the coming weeks.  

For this week's analysis, we have used a model that includes adjustments for this period. The estimates are based on fewer data points meaning there is greater uncertainty for these estimates. While we routinely update our estimates when more test results are available, results will be updated as soon as data for this period are available. 

We present weekly modelled antibody estimates for adults by country, and grouped age for England, Wales, Northern Ireland and Scotland, as well as by regions in England. Single year of age analysis has not been updated in this week's publication. We also include estimates of adults who reported they have received one or more doses of a COVID-19 vaccine, and of adults who reported they have received two or more doses of a COVID-19 vaccine (including booster doses).

There is a clear pattern between vaccination and testing positive for COVID-19 antibodies but the detection of antibodies alone is not a precise measure of the immunity protection given by vaccination. Further information is available on antibody levels post vaccination from research partners at the University of Oxford.

Modelled vaccine estimates are produced to provide context alongside our antibodies estimates and do not replace the official government figures on vaccinations, which are a more precise count of total vaccines issued. While we would expect the overall trend of our estimated number of people who have received vaccines to increase, it is possible that in some weeks, the estimate may remain the same or decrease as a result of sampling variability (for example, we may have a lower number of participants recording a vaccination in the latest week compared with an earlier week).

National Immunisation Management System (NIMS) administrative data are used to validate CIS self-reported records of vaccination for England. The equivalent of this is currently not included for other countries, meaning the estimates for Wales, Scotland and Northern Ireland are produced just from CIS self-reported records of vaccination.

Our vaccination estimates are explained in more detail in Section 9: Measuring the data.

Antibody positivity is defined by having a fixed concentration of antibodies in the blood. A negative test result occurs if there are no antibodies, or if antibody levels are too low to reach a threshold at the time of testing. The threshold we use is 42 nanograms per millilitre (ng/ml). This is the threshold that the test is CE marked against providing greater than 99% sensitivity and specificity. Following infection or vaccination, antibody levels can vary and sometimes increase but may still be below the level identified as positive in our test and other tests, and so give a negative result.

Most people who are vaccinated will retain a higher antibody level than before vaccination even after dropping below the standard threshold value. A negative result does not mean that the antibody level is at zero, nor that a person has no protection against COVID-19, as an immune response does not rely on the presence of antibodies alone.

There are other parts of the immune system that will offer protection, for example, a person's T-cell response. This will not be detected by blood tests for antibodies. A person's immune response is affected by a number of factors, including health conditions and age.

Equally, antibody levels are expected to decrease over time irrespective of vaccination or natural infection, especially when exposure to the virus is reduced. This is because our bodies stop making antibodies when they are not needed.

Our blog on antibodies and immunity gives further information on the link between antibodies and immunity and the vaccine programme. Our blog on vaccine effectiveness provides information on the effectiveness of vaccinations against Alpha and Delta variants, which is based upon the research conducted by partners from the University of Oxford.

Our methodology article provides further information around the survey design, how we process data, and how data are analysed. The study protocol specifies the research for the study. The Coronavirus (COVID-19) Infection Survey QMI details the strength and limitations of the data.

In the week beginning 18 October, there was an increase in the percentage of people testing positive for antibodies in the most recent weeks for all nations except Northern Ireland, although estimates for Northern Ireland remain high.

In the week beginning 18 October 2021, our estimates suggest the percentage of adults who reported they had received at least one dose of a coronavirus (COVID-19) vaccine remained high, ranging from 93.4% to 95.6% of adults across the UK. The estimated percentage of adults in the UK who had received two or more doses of a COVID-19 vaccine also remained high or continued to increase, ranging from 82.5% to 89.4% of adults across the UK in the week beginning 18 October 2021. These vaccination estimates will differ from official figures as explained in Section 1.

Figure 1: The percentage of adults testing positive for COVID-19 antibodies increased in the most recent weeks up to 18 October 2021 for all nations except Northern Ireland, although estimates for Northern Ireland remain high

Modelled percentage of adults who tested positive for antibodies to SARS-CoV-2, who have received one or more doses of a COVID-19 vaccine, and two or more doses of a COVID-19 vaccine, UK countries, 7 December 2020 to 24 October 2021

Notes:

1. All results are provisional and subject to revision.
2. Estimates for dates between 27 August 2021 and 5 September 2021 use partial data. See "Data in this bulletin" in Section 1 for more information.
3. These statistics refer to antibody tests and reported vaccinations for individuals living in private households. These figures exclude individuals in hospitals, care homes and/or other communal establishments.
4. All estimates are subject to uncertainty, given that a sample is only part of the wider population. A credible interval gives an indication of the uncertainty of an estimate from data analysis.
5. The denominators used for both antibodies and vaccinations are the total people in the sample at that particular time point, then post-stratified by the mid-year population estimate.
6. Vaccination data for Northern Ireland start later than the other countries, from 21 December 2020.
7. Our estimates of vaccination are provided for context alongside our antibodies estimates but are likely to be different from the official figures. The daily official government figures provide the recorded actual numbers of vaccines against SARS-CoV-2 issued.

Download the data

The estimated percentage of adults who have received at least one dose of a coronavirus (COVID-19) vaccine, remained at a high level across all regions of England in the week beginning 18 October 2021. Regional data can be found in our accompanying dataset.

In the week beginning 18 October 2021, the percentage of adults testing positive for coronavirus (COVID-19) antibodies remained high. In England, Wales and Scotland, the percentage of people testing positive for antibodies increased in most age groups. In Northern Ireland, the percentage of people testing positive for antibodies increased in younger age groups and showed signs of a slow decline in older age groups.

Many of the participants who have tested negative for antibodies in the most recent week received their vaccinations in the early stages of the vaccination programme. Along with age, the early stages of the vaccination programme prioritised individuals on a range of factors (such as long-term health conditions and type of employment) and this may help to explain the declining trend appearing in different age groups.

It is important to note that antibody positivity is defined by a fixed concentration of antibodies in the blood.

Most people who are vaccinated will retain higher antibody levels than before vaccination but may have antibodies below the threshold used at the time of testing. This does not mean that these people have no protection against new infection.

More information can be found in our blog about antibodies and immunity and in Section 2 of this bulletin.

We estimate that the antibody positivity for those aged 16 to 24 years increased steadily across the UK, with estimates ranging between 96.0% and 97.4% across the UK, for the week beginning 18 October 2021.

The percentage of adults who reported they have received at least one dose of a COVID-19 vaccine is lowest in the younger age groups but is increasing.

Across the UK, the estimated percentage of adults aged 16 to 24 years who have received one or more doses of a COVID-19 vaccine ranged from 80.2% to 83.7%; this has increased sharply since the end of June 2021. These estimates of vaccination are based on reported vaccination status in our survey and are likely to be different from the official vaccination figures for the UK.

Modelled antibody estimates and modelled vaccine estimates relate to the adult population (aged 16 years and over) who live in private households. The true figure among the older age groups in the population may be different. In England, an estimated 90% of people aged 80 years and over live in private households and 10% live in other communal establishments such as care homes.

We are seeing increasing numbers of tests close to the threshold for detection so we can expect to see fluctuations in our estimates of antibody positivity over time. We will continue to monitor the longer-term trend, and caution should be applied if comparing antibody levels between specific time periods.

Figure 2: The percentage of adults testing positive for COVID-19 antibodies continued to remain high in the UK, but showed signs of a slow decline in older adults in Northern Ireland

Modelled percentage of adults who tested positive for antibodies to SARS-CoV-2, who have received one or more doses of a COVID-19 vaccine, and those who have received two or more doses of a COVID-19 vaccine, by grouped age, UK countries, 7 December 2020 to 24 October 2021

Notes:

1. All results are provisional and subject to revision.
2. Estimates for dates between 27 August 2021 and 5 September 2021 use partial data. See "Data in this bulletin" in Section 1 for more information.
3. These statistics refer to antibody tests and reported vaccinations for individuals living in private households. These figures exclude individuals in hospitals, care homes and/or other communal establishments.
4. In Northern Ireland, the number of people sampled is low compared with England, Wales and Scotland; therefore, people aged 50 to 69 years are included in the same age group, and those aged 70 years and over are included in the same age group.
5. All estimates are subject to uncertainty, given that a sample is only part of the wider population. A credible interval gives an indication of the uncertainty of an estimate from data analysis.
6. The denominators used for both antibodies and vaccinations are the total people in the sample at that particular time point, then post-stratified by the mid-year population estimate.
7. Our estimates of vaccination are provided for context alongside our antibodies estimates but are likely to be different from the official figures. The daily official government figures provide the recorded actual numbers of vaccines against SARS-CoV-2 issued.

Download the data

The Coronavirus (COVID-19) Infection Survey analysis was produced by the Office for National Statistics (ONS) in collaboration with our research partners at the University of Oxford, the University of Manchester, UK Health Security Agency and Wellcome Trust. Of particular note are:

• Sarah Walker – University of Oxford, Nuffield Department for Medicine: Professor of Medical Statistics and Epidemiology and Study Chief Investigator
• Koen Pouwels – University of Oxford, Health Economics Research Centre, Nuffield Department of Population Health: Senior Researcher in Biostatistics and Health Economics
• Thomas House – University of Manchester, Department of Mathematics: Reader in Mathematical Statistics

Confidence interval

A confidence interval gives an indication of the degree of uncertainty of an estimate, showing the precision of a sample estimate. The 95% confidence intervals are calculated so that if we repeated the study many times, 95% of the time the true unknown value would lie between the lower and upper confidence limits. A wider interval indicates more uncertainty in the estimate. Overlapping confidence intervals indicate that there may not be a true difference between two estimates.

For more information, see our methodology page on statistical uncertainty.

Credible interval

A credible interval gives an indication of the uncertainty of an estimate from data analysis. A 95% credible interval is calculated so that there is a 95% probability of the true value lying in the interval.

More information on measuring the data is available in the Coronavirus (COVID-19) Infection Survey bulletin.

Our methodology article provides further information around the survey design, how we process data, and how data are analysed. The Coronavirus (COVID-19) Infection Survey QMI explains the strengths and limitations of the data, methods used, and data uses and users.

Vaccination estimates

While the daily official government figures provide the recorded actual numbers of vaccines against SARS-CoV-2 issued, our vaccination estimates are likely to be different from the official figures. This is because they are estimates based on a sample survey of reported vaccine status and are provided for context alongside our antibodies estimates.

We control for the effect of ethnicity by post-stratifying our analysis by White and non-White ethnic groups, rather than individual ethnicities, because of our current sample size. This could result in differences between our survey estimates and the government figures in the numbers of vaccines received for some ethnic minority groups.

Importantly, our survey collects information from the population living in private households and does not include people living in communal establishments such as care homes, hospitals or prisons.

The value of showing our estimates of vaccines alongside our estimates of people testing positive for antibodies is to illustrate the relationship between the two.

Differences between official figures and the estimates from this survey differ in scale across each of the four UK nations (some survey estimates are closer to the official reported figures than others) because of differences in reporting dates and the inclusion of National Immunisation Management System (NIMS) data for England. In addition, our sampling method for Northern Ireland is different to the other nations, inviting only people who have previously participated in a Northern Ireland Statistics and Research Agency (NISRA) survey, which could result in a sample of individuals who are more likely to get vaccinated. This should be taken into consideration if comparing vaccine and antibody estimates across the four nations, as vaccine status and antibody positivity are related.

In addition, as our analysis develops, our survey-based estimates will enable possible future analysis of people who have received a vaccine with other characteristics collected in the survey.

More information on strengths and limitations of the data is available in the Coronavirus (COVID-19) Infection Survey QMI and in the Coronavirus (COVID-19) Infection Survey bulletin.