Coronavirus (COVID-19): ONS Infection Survey – antibody data – 14 April 2021

This data refers to the presence of antibodies to SARS-CoV-2 within the community population aged 16 and over; community in this instance refers to private residential households, and it excludes those in hospitals, care homes and/or other institutional settings. SARS-CoV-2 is the scientific name given to the specific virus that causes COVID-19. All results are provisional and subject to revision.

This article presents an analysis of antibody prevalence, which can be used to identify individuals who have had COVID-19 in the past or who have developed antibodies as a result of vaccination. The article is based on findings from the Office for National Statistics (ONS) COVID-19 Infection Survey. 

It is important to draw the distinction between testing positive for antibodies and having immunity. Following infection or vaccination, antibody levels can vary and sometimes increase but are still below the level identified as “positive” in our test, and other tests. This does not mean that a person has no protection against coronavirus (COVID-19) since an immune response does not rely on the presence of antibodies alone. We also do not yet know exactly how much antibodies need to rise to give protection. A person’s ‘T cell’ response will provide protection but is not detected by blood tests for antibodies. 

Estimates on the proportion of people in the community population that have received at least one dose of a vaccine against SARS-CoV-2 are also reported, which helps show a clear pattern between vaccination and testing positive for antibodies.

ONS have recently changed the way they produce estimates of antibody positivity. Previously they presented weighted estimates for 28-day periods of antibody positivity for England, Wales, Northern Ireland and Scotland, with fortnightly updates on antibody data. The first article using this new methodology was published on 30 March 2021.

Estimates are now based on a model where England, Wales and Scotland are included together in a spatial-temporal model with Northern Ireland being modelled separately. This reflects the geography of the four countries with Northern Ireland not sharing a land border with Great Britain. The geo-spatial model incorporates physical land distance between regions. This new modelling approach means that ONS are now able to provide more timely weekly estimates given the rapid vaccination rollout, whilst still adjusting to make the estimates representative of the population. The data is modelled on standardised Monday-Sunday surveillance weeks and data from 7 December 2020 is presented. The latest week’s modelled estimate is subject to more uncertainty as it is an incomplete week of data and therefore more likely to change when more data become available.  Further information on this method to model antibodies can be found in ONS updated methods article.

ONS also present data on the estimated percentage of people who have received one or more doses of a COVID-19 vaccination since 14 December 2020. Vaccination rates were too low to model any earlier. The estimates of the percentage of people vaccinated are based on modelling of the people visited in the COVID-19 Infection Survey in the community in a particular time period. These estimates are then adjusted (post-stratified) to be representative of the population.

It is important to note that this release contains estimates based on self-reported vaccines from the ONS COVID-19 Infection Survey and should not be used to track the progress of the vaccine rollout. Please see the latest daily vaccination data on the UK coronavirus dashboard to understand the progress of the vaccination programme across the UK.

Survey estimates refer to the community population, by which we mean private households only and excludes those in hospitals, care homes or other institutional settings. This data does not include those that live in care homes, one of the priority groups identified by the Joint Committee on Vaccination and Immunisation (JCVI).

There is high uncertainty around COVID-19 Infection Survey estimates due to the relatively small number of people included in this analysis, so caution should be taken in interpreting the results.

Further information on the methodology can be found at the end of this release. 

Antibody data is a week behind vaccination data as there is a time lag on when antibody data is received, whereas vaccine data is self-reported and more readily available.

Likelihood of testing positive for COVID-19 antibodies in Scotland

In the week 22 to 28 March 2021, it is estimated that 46.0% of the community population in Scotland aged 16 and over would have tested positive for antibodies to SARS-CoV-2 from a blood sample. A 95% credible interval for this estimate is 40.3% to 51.6%.

Modelled estimates suggest that antibody positivity levelled off in the week 22 to 28 March 2021.

Modelled weekly estimates of the percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample, are displayed in Figure 1 as estimates for the midpoint of the week.

Figure 1: Modelled weekly percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample, from 7 December 2020 to 28 March 2021, including 95% credible intervals

Age analysis on the likelihood of testing positive for COVID-19 antibodies in Scotland

In the week 22 to 28 March 2021, the modelled percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample for those:

• aged 80 years and over was 66.6% (95% credible interval: 55.6% to 75.0%);
• aged 75 to 79 years was 69.4% (95% credible interval: 60.4% to 77.4%);
• aged 70 to 74 years was 78.0% (95% credible interval: 70.6% to 83.8%);
• aged 65 to 69 years was 82.9% (95% credible interval: 76.9% to 87.8%);
• aged 60 to 64 years was 59.1% (95% credible interval: 50.2% to 67.9%) and
• aged 50 to 59 years was 48.1% (95% credible interval: 39.5% to 57.3%).

The modelled percentage of people testing positive for antibodies for those aged 16 to 49 years ranged from 27.4% to 31.4%

Figure 2: Modelled percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample, by age group, in the week 22 to 28 March 2021, including 95% credible intervals

Figure 3 shows the modelled weekly estimate of the percentage of people testing positive for antibodies to SARS-CoV-2 from 7 December 2020 to 28 March 2021 by age group, showing the trend over time. In recent weeks, antibody positivity estimates have decreased amongst older age groups, likely because the data does not yet show the impact of second doses of COVID-19 vaccinations. Antibody positivity is still lowest in the younger age groups, due to the prioritisation of vaccinations in older age groups.

Modelled weekly estimates of the percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample by age group are displayed in Figure 3 as estimates for the midpoint of the week.

Figure 3: Modelled weekly percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample, by age group, from 7 December 2020 to 28 March 2021, including 95% credible intervals

 

Modelled daily estimates of antibody positivity by single year of age in the most recent weeks can also be found in the accompanying dataset and visualisation published by the ONS.

The methodology used to produce the daily modelled estimates is different to that used to produce the weekly estimates and as such, these figures are not directly comparable. However, the broad pattern across age groups appears similar.

Comparison of modelled estimates of the proportion of people who have received one or more doses of a COVID-19 vaccination with the likelihood of testing positive for antibodies to SARS-CoV-2 in Scotland

It is important to note that this section contains estimates based on self-reported vaccines from the ONS COVID-19 Infection Survey and should not be used to track the progress of the vaccine rollout. Please see the latest daily vaccination data on the UK coronavirus dashboard to understand the progress of the vaccination programme across the UK.

Survey estimates refer to the community population, by which we mean private households only and excludes those in hospitals, care homes or other institutional settings. This data does not include those that live in care homes, one of the priority groups identified by the Joint Committee on Vaccination and Immunisation (JCVI).

Further information on the methodology can be found at the end of this release. 

Estimates from the survey are not the same as data reported from the UK coronavirus dashboard which includes daily data for the UK and each constituent country on the actual number of people who have received a COVID-19 vaccination based on individual vaccination records (administrative data held by each nation). The UK coronavirus dashboard figures include those in care homes, hospitals and other communal establishments.

The estimates produced from the survey are helpful to compare with other characteristics, such as testing positive for antibodies and further analysis will be produced as more data become available.

Modelled estimates suggest there has been an increase in the proportion of people who have been vaccinated in the week ending 3 April 2021 and that antibody positivity levelled off in the week ending 28 March 2021 in Scotland. There is a clear pattern between vaccination and testing positive for COVID-19 antibodies as shown in Figure 4.

Modelled weekly estimates are displayed in Figure 4 as estimates for the midpoint of the week.

Figure 4: Modelled weekly estimate of percentage of people that have received one or more doses of a COVID-19 vaccine, from 14 December 2020 to 3 April 2021 and modelled weekly percentage of people testing positive for antibodies to SARS-CoV-2 from a blood sample, from 7 December 2020 to 28 March 2021, including 95% credible intervals

Modelled weekly estimates of the percentage of people that have received one or more doses of a COVID-19 vaccine by age group can also be found in the full article published by the Office for National Statistics on 14 April 2021.

Methodology and further information

The analysis in this section of the article is based on blood test results taken from a randomly selected subsample of individuals aged 16 years and over, which are used to test for antibodies against SARS-CoV-2. This can be used to identify individuals who have had the infection in the past or have developed antibodies as a result 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 remain in the blood at low levels, although these levels can decline over time to the point that tests can no longer detect them. Having antibodies can help to prevent individuals from getting the same infection again.

The presence of antibodies is measured to understand who has had COVID-19 in the past and the impact of vaccinations. Once infected, the length of time antibodies remain at detectable levels in the blood is not fully known. It is also not yet known how having detectable antibodies, now or at some time in the past, affects the chance of getting COVID-19 again, since other parts of the immune system (T cell response) will offer protection.

The full article on antibody and vaccination data published by the Office for National Statistics on 14 April 2021, which includes antibody information for England, Wales and Northern Ireland, is available.

More information about the COVID-19 Infection Survey in Scotland can be found on the information page on the Scottish Government website, and previous COVID-19 Infection Survey data for Scotland can be found in this collection.

Methodology notes:

1. The model used to provide these estimates is a Bayesian model: these provide 95% credible intervals. A credible interval gives an indication of the uncertainty of an estimate from data analysis. 95% credible intervals are calculated so that there is a 95% probability of the true value lying in the interval. A wider interval indicates more uncertainty in the estimate.
2. National Immunisation Management System (NIMS) administrative data is used to validate COVID-19 Infection Survey self-reported records of vaccination for England. The equivalent of this is currently not included for other countries, meaning the estimates for Wales, Northern Ireland and Scotland are produced only from COVID-19 Infection Survey self-reported records of vaccination.
3. The denominators used for vaccinations are the total people in the sample at that particular time point, which are then post-stratified by the mid-year population estimate.
4. There is high uncertainty around COVID-19 Infection Survey estimates due to the relatively small number of people included in this analysis, so caution should be taken in interpreting the results.