Coronavirus (COVID-19) Infection Survey, antibody and vaccination data, UK: 23 December 2021

The following statistics are the percentage of the adult population aged 16 years and over, that would have tested positive on a blood test for antibodies against SARS-CoV-2, the specific virus that causes coronavirus (COVID-19). This suggests that they have had the infection in the past or have been vaccinated.

• In England, it is estimated that 95.0% of the adult population (95% credible interval: 94.2% to 95.7%) would have tested positive for antibodies against SARS-CoV-2 in the week beginning 29 November 2021.

• In Wales, it is estimated that 93.6% of the adult population (95% credible interval: 92.3% to 94.8%) would have tested positive for antibodies against SARS-CoV-2 in the week beginning 29 November 2021.

• In Northern Ireland, it is estimated that 95.3% of the adult population (95% credible interval: 93.9% to 96.5%) would have tested positive for antibodies against SARS-CoV-2 in the week beginning 29 November 2021.

• In Scotland, it is estimated that 95.0% of the adult population (95% credible interval: 94.0% to 95.9%) would have tested positive for antibodies against SARS-CoV-2 in the week beginning 29 November 2021.

• The percentage of the adult population who would have tested positive for antibodies against SARS-CoV-2 has increased among those aged 65 years and over in England, Wales and Scotland, and among those aged 70 years and over in Northern Ireland, likely as a result of the vaccination booster programme.

• Across all UK countries, the percentage of adults who have reported they have received three doses of a COVID-19 vaccine has increased rapidly since September 2021, in line with the vaccine programmes.

About this bulletin

This bulletin presents the latest estimates of the number of adults who would test positive for antibodies to SARS-CoV-2 for England, Wales, Northern Ireland, and Scotland based on findings from the Coronavirus (COVID-19) Infection Survey in the UK. People who test positive for antibodies are those who have experienced previous infection and/or vaccination. We also present data, for the first time, on the percentage of people aged 16 years and over who reported that they have received three doses of a COVID-19 vaccine since 13 September 2021.

Modelled vaccination estimates for one or more doses and two or more doses of a COVID-19 vaccine require additional quality assurance and have not been updated in our bulletin this time. We will reintroduce these vaccine estimates as soon as possible. Official government figures on vaccinations are available.

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. 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.

Our antibodies and vaccination estimates are based on the data we collect from people visited in the Coronavirus (COVID-19) Infection Survey (CIS). We present weekly modelled antibody estimates and vaccine estimates for adults by country, and grouped age for England, Wales, Northern Ireland and Scotland, as well as antibody estimates by regions in England. Further information on our method to model antibodies and vaccinations can be found in our methods article.

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 reported they 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 from CIS self-reported records of vaccination only.

Our vaccination estimates are explained in more detail in Section 10: 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 coronavirus (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. It is too early to provide information on the effectiveness of vaccinations against the Omicron variant.

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 29 November 2021, the percentage of adults testing positive for coronavirus (COVID-19) antibodies remained high across the UK.

Figure 1: The percentage of adults testing positive for COVID-19 antibodies remained high across UK countries in the week beginning 29 November 2021

Modelled percentage of adults: who tested positive for antibodies to SARS-CoV-2, UK countries, 7 December 2020 to 5 December 2021

Notes:

1. All results are provisional and subject to revision.
2. These statistics refer to antibody tests for individuals living in private households. These figures exclude individuals in hospitals, care homes and/or other communal establishments.
3. 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.
4. The denominators used for antibodies are the total people in the sample at that particular time point, then post-stratified by the mid-year population estimate.

Download the data

.xlsx

The estimated percentage of adults who reported they have received three doses of a COVID-19 vaccine has increased rapidly since September 2021. Modelled vaccination estimates since 13 September 2021 can be found in the Coronavirus (COVID-19) Infection Survey: antibody and vaccination dataset. These vaccination estimates will differ from official figures as explained in Section 1. Table 2 shows the latest estimated percentage of adults who reported they have received three vaccination doses in the week beginning 29 November 2021.

The percentage of adults testing positive for coronavirus (COVID-19) antibodies remained at high levels across all regions of England in the week beginning 29 November 2021. Regional data can be found in Tables 1b and 1f in the Coronavirus (COVID-19) Infection Survey: antibody and vaccination dataset.

In the week beginning 29 November 2021, the percentage of adults testing positive for coronavirus (COVID-19) antibodies remained high across all age groups across the UK. There were increases in antibody positivity among those aged 65 years and over in England, Wales and Scotland, and among those aged 70 years and over in Northern Ireland. There has been an increase in antibody positivity in those aged 65 years and over across the UK since early October 2021, likely as a result of the vaccination booster programme.

Modelled vaccination estimates from those who reported they have received three doses of a COVID-19 vaccine by grouped age are provided in the Coronavirus (COVID-19) Infection Survey: antibody and vaccination dataset. These estimates of vaccination are based on reported vaccination status in our survey and are likely to be different from the official figures for the UK.

Figure 2: The percentage of adults testing positive for COVID-19 antibodies increased among those aged 65 years and over in England, Wales and Scotland, and among those aged 70 years and over in Northern Ireland in the week beginning 29 November 2021

Modelled percentage of adults: who tested positive for antibodies to SARS-CoV-2, by age group, UK countries, 7 December 2020 to 5 December 2021

Notes:

1. All results are provisional and subject to revision.
2. These statistics refer to antibody tests for individuals living in private households. These figures exclude individuals in hospitals, care homes and/or other communal establishments.
3. 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.
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 antibodies are the total people in the sample at that particular time point, then post-stratified by the mid-year population estimate.

Download the data

.xlsx

Modelled daily estimates of coronavirus (COVID-19) antibody positivity by single year of age for England, Wales, Northern Ireland and Scotland separately, between 25 October and 5 December 2021 can be found in the Coronavirus (COVID-19) Infection Survey: antibody and vaccination dataset. These modelled data are produced using a different method to the weekly modelled estimates presented in Section 5 and so cannot be compared.

This more granular analysis shows a similar pattern to our grouped age analysis, with antibody positivity remaining high across the UK.

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

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 vaccinations. 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.

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.

Reference dates

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 estimates for the most recent week in this publication includes data from 29 November to 5 December 2021. The vaccination estimates for those who reported they have received three doses of a coronavirus (COVID-19) vaccine for the most recent week in this publication includes data from 29 November to 5 December 2021. Modelled vaccination estimates for one or more doses and two or more doses of a COVID-19 vaccine for England, Wales, Northern Ireland and Scotland have not been updated this time because of additional quality assurance required.

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

Our methodology article provides further information around the survey design, how we process data, and how data are analysed. The Quality and Methodology Information 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 other 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.

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