Coronavirus (COVID-19): ONS Infection Survey – antibody data for Scotland – 6 April 2022

ONS Coronavirus (COVID-19) Infection Survey – Antibody Data – 6 April 2022

This publication 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 findings presented in this publication are based on data from the Office for National Statistics (ONS) COVID-19 Infection Survey.

The COVID-19 Infection Survey aims to measure:

• how many people test positive for COVID-19 infection at a given point in time, regardless of whether they report experiencing coronavirus symptoms
• the average number of new infections per week over the course of the study
• the number of people who test positive for antibodies, to indicate how many people are ever likely to have had the infection or have been vaccinated

All results are provisional and subject to revision.

The Office for National Statistics (ONS) publish estimates for England, Wales, Northern Ireland and Scotland on their website.

The underlying data displayed in the charts in this publication is available in the reference tables on the ONS website.

Main Points

In Scotland, 98.9% of the adult (aged 16+) population living in private residential households (95% credible interval: 98.5% to 99.1%) are estimated to have antibodies against SARS-CoV-2 at the 179 ng/ml threshold, from a blood sample in the week beginning 14 March 2022.

The estimated percentage of the adult (aged 16+) population living in private residential households in Scotland testing positive for antibodies against SARS-CoV-2 at the 179 ng/ml threshold ranged from 98.0% for those aged 80 years and over (95% credible interval: 96.3% to 98.9%) and 99.4% for those aged 65 to 69 years (95% credible interval: 99.2% to 99.6%) and 70 to 74 years (95% credible interval: 99.1% to 99.6%), in the week beginning 14 March 2022.

The percentage of children living in private residential households in Scotland who are estimated to have antibodies against SARS-CoV-2 at the 179 ng/ml threshold was 85.3% for those aged 8 to 11 years (95% credible interval: 75.5% to 91.8%) and 97.2% for those aged 12 to 15 years (95% credible interval: 94.6% to 98.6%), from a blood sample in the week beginning 14 March 2022.

As detailed by the ONS, 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.

The antibody threshold of 179 ng/ml is higher than the previously reported standard threshold of 42 ng/ml and will provide the earliest signs of any changes in antibody levels. Academic research on antibody thresholds using data from when the Delta variant was the dominant strain indicate that the higher antibody threshold is needed to provide protection from new COVID-19 infections for those who are vaccinated.

Information on this release

The most recent antibody estimates in this publication include data from 14 to 20 March 2022.

In this publication, the following terminology is used:

• Antibodies - the presence of antibodies to SARS-CoV-2 is measured in the population of adults (aged 16+) and children (aged 8 to 15) living in private residential households (excludes those in hospitals, care homes and/or other institutional settings) to understand who has had 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. Having antibodies can help to prevent individuals from getting the infection and if they do get infected, they are less likely to have severe symptoms. It does not guarantee that an individual cannot be infected with COVID-19. 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.

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 previously reported standard antibody threshold was 42 nanograms per millilitre (ng/ml). This is the threshold that the test is CE marked against and approved by the Medicines and Healthcare products Regulatory Agency, providing greater than 99% sensitivity and specificity in identifying people who have had a COVID-19 infection before (“natural immunity”) from people who have not. A negative result means that detected antibody levels are below this threshold and does not necessarily mean that a person has no antibodies or immune protection.

The previously reported standard antibody threshold of 42 ng/ml was determined prior to the development of COVID-19 vaccinations. As the pandemic and vaccinations have evolved, the ONS have reviewed the way that it presents information about antibody levels. The ONS have introduced the 179 ng/ml antibody threshold associated with a higher concentration of antibodies to estimate the percentage of adults who are likely to have a strong antibody response to protect from getting a new COVID-19 infection. This is based upon research by academic partners. A previous COVID-19 infection typically results in a stronger immune response than vaccination, and to get a similar level of protection from vaccination alone, a higher concentration of antibodies is needed.

The higher antibody threshold is 179 ng/ml and was identified as providing a 67% lower risk of getting a new COVID-19 infection with the Delta variant after two vaccinations with either Pfizer or AstraZeneca vaccines, compared with someone who was unvaccinated and had not had COVID-19 before. This higher antibody threshold was identified from comparing how the risk of new COVID-19 infections with the most common COVID-19 variant at the time of the research, the Delta variant, varied across different antibody levels. The higher antibody threshold will provide the earliest signs of any changes in antibody levels. It is unlikely that this antibody threshold will provide equivalent protection against the Omicron variant and analyses of the effectiveness of vaccinations against the Omicron variant are ongoing.

In this publication, we present the estimated percentage of adults with antibodies to SARS-CoV-2 at the higher antibody threshold of 179 ng/ml, first introduced to these publications on 13 January 2022. The figures also visualise the estimated percentage of adults with antibodies at the previously reported standard threshold of 42 ng/ml. Additional breakdowns of the data at both thresholds are available in the Coronavirus (COVID-19) Infection Survey: antibody and vaccination dataset.

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 estimates: likelihood of testing positive for antibodies against SARS-CoV-2

In Scotland, 98.9% of the adult (aged 16+) population living in private residential households (95% credible interval: 98.5% to 99.1%) are estimated to have antibodies against SARS-CoV-2 at the 179 ng/ml threshold, from a blood sample in the week beginning 14 March 2022, suggesting that they had the infection in the past or have been vaccinated. Please see note 1 for information on credible intervals.

Modelled weekly estimates of the percentage of people testing positive for coronavirus (COVID-19) antibodies by the antibody threshold of 179 ng/ml and the previously reported standard threshold of 42 ng/ml from a blood sample, are displayed in Figure 1 as estimates for the midpoint of the week.

Figure 1: Modelled weekly percentage of people in the adult population (aged 16+) living in private residential households testing positive for antibodies to SARS-CoV-2 from a blood sample, from 7 December 2020 to the week beginning 14 March 2022, including 95% credible intervals

 

Antibody estimates by age group for adults (aged 16+): likelihood of testing positive for antibodies against SARS-CoV-2

In recent weeks, antibody positivity in adults (aged 16+) has continued to remain high across all age groups in Scotland.

The estimated percentage of the adult (aged 16+) population living in private residential households in Scotland testing positive for antibodies against SARS-CoV-2 at the 179 ng/ml threshold ranged from 98.0% for those aged 80 years and over (95% credible interval: 96.3% to 98.9%) and 99.4% for those aged 65 to 69 years (95% credible interval: 99.2% to 99.6%) and 70 to 74 years (95% credible interval: 99.1% to 99.6%), in the week beginning 14 March 2022.

Figure 2 shows the modelled weekly estimate of the percentage of adults living in private residential households testing positive for COVID-19 antibodies at the antibody threshold of 179 ng/ml and the previously reported standard threshold of 42 ng/ml, from 7 December 2020 to the week beginning 14 March 2022, by age group, showing the trend over time.

Across the UK, antibody waning is more evident in those aged 50 years and over when using the higher antibody threshold of 179 ng/ml in comparison with the standard antibody threshold of 42 ng/ml, between May 2021 and September 2021. Between early-October and late-December 2021, there was a rapid increase in antibody levels at the antibody threshold of 179 ng/ml across the UK among those aged 50 years and over, likely as a result of the vaccination booster programme. Antibody levels at the higher antibody threshold also increased relatively fast in younger adults between late-November and mid-January, and have since remained high for all adult age groups.

It is important to note that this analysis defines antibody positivity by a fixed amount of antibodies in blood samples. Most people who are vaccinated will retain higher antibody levels than before vaccination but may have a lower number of antibodies than this threshold at the time of testing. This does not mean that these people have no protection against new infection. Please read the ONS blog on Antibodies and Immunity for more information. Estimates vary slightly week on week due to sampling variability.

Figure 2: Modelled weekly percentage of people in the adult (16+) population living in private residential households testing positive for antibodies to SARS-CoV-2 from a blood sample, by age group, from 7 December 2020 to the week beginning 14 March 2022, including 95% credible intervals

 

Previous modelled weekly estimates of the percentage of adults (aged 16+) that have received three doses of a COVID-19 vaccine by age group can also be found in the full bulletin published by the ONS on 23 February 2022. These estimates of vaccination are based on reported vaccination status in the survey and are likely to be different from the official figures.

Antibody estimates by age group for children (aged 8 to 15): likelihood of testing positive for antibodies against SARS-CoV-2

The ONS have collected samples from children aged between 8 and 15 years since 29 November 2021 to test for COVID-19 antibodies. The antibody estimates have been produced using the same models as the estimates for those aged 16 years and over but have been post-stratified separately to be representative of the population.

The percentage of children living in private residential households in Scotland who are estimated to have antibodies against SARS-CoV-2 at the 179 ng/ml threshold was 85.3% for those aged 8 to 11 years (95% credible interval: 75.5% to 91.8%) and 97.2% for those aged 12 to 15 years (95% credible interval: 94.6% to 98.6%), from a blood sample in the week beginning 14 March 2022.

The number of children sampled is lower compared to the sample size for those aged 16 and over. This means there is a higher degree of uncertainty in estimates for those under 16 years when the analysis splits the sample into smaller groups (for example, further age groups) as indicated by larger credible intervals.

Figure 3 shows the modelled weekly estimate of the percentage of children (under 16 years) living in private residential households testing positive for COVID-19 antibodies at the antibody threshold of 179 ng/ml and the previously reported standard threshold of 42 ng/ml, from 29 November 2021 to the week beginning 14 March 2022, by age group, showing the trend over time.

Figure 3: Modelled weekly percentage of children (under 16 years) living in private residential households testing positive for antibodies to SARS-CoV-2 from a blood sample, by age group, from 29 November 2021 to the week beginning 14 March 2022, including 95% credible intervals

 

Previous modelled weekly estimates of the percentage of children aged 12 to 15 years who reported that they have received one or more and two or more COVID-19 vaccinations since 29 November 2021 can also be found in the full bulletin published by the ONS on 23 February 2022. These estimates of vaccination are based on reported vaccination status in the survey and are likely to be different from the official figures.

Methodology and further information

1. The analysis presented in this publication is based on blood test results taken from a randomly selected subsample of individuals aged 16 years and over (living in private residential households), 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.
2. The presence of antibodies is measured to understand who has had 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. Having antibodies can help to prevent individuals from getting the infection and if they do get infected, they are less likely to have severe symptoms. It does not guarantee that an individual cannot be infected with COVID-19.
3. 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. It is also not yet known how having detectable antibodies, now or at some time in the past, affects the chance of becoming infected or experiencing symptoms, as other parts of the immune system (T cell response) will offer protection. Antibody positivity is defined by a fixed amount of antibodies in the blood. A negative test result will occur if there are no antibodies or if antibody levels are too low to reach this threshold.
4. 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 this test, and other tests. This does not mean that a person has no protection against 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. A person's immune response is affected by a number of factors, including health conditions and age. Additional information on the link between antibodies and immunity and the vaccine programme can be found on the ONS blog.
5. The daily official government figures provide the recorded actual numbers of vaccines against SARS-CoV-2 issued; the vaccination estimates from the COVID-19 Infection Survey are likely to be different from the official figures and should not be used to track the progress of the vaccine rollout. Please see the latest daily official government figures on vaccination data on the UK coronavirus dashboard to understand the progress of the vaccination programme across the UK. This is because they are estimates based on a sample survey of reported vaccine status and are provided for context alongside antibodies estimates. Importantly, the 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. Those that live in care homes were one of the priority groups identified by the Joint Committee on Vaccination and Immunisation (JCVI). The value showing estimates of vaccines alongside estimates of people testing positive for antibodies is to illustrate the relationship between the two.
6. Differences between official figures and the estimates from this survey differ in scale across each of the four nations (some survey estimates are closer to the official reported figures than others) due to differences in reporting dates and the inclusion of National Immunisation Management System (NIMS) data for England. In addition, the sampling method for Northern Ireland is different to the other nations, inviting only people that have previously participated in a Northern Ireland Statistics and Research Agency (NISRA) survey, which could result in a sample of individuals that are more likely to get vaccinated. This should be taken into consideration if comparing vaccine and antibody estimates across the four nations, since vaccine status and antibody positivity are related. In addition, as the analysis develops, the survey-based estimates will enable possible future analysis of people who have received a vaccine with other characteristics collected in the survey. ONS have recently published a blog which provides more information on what the ONS can tell you about the COVID-19 vaccine programme.
7. Additionally, ONS have made a blog post providing 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.
8. The full bulletin on antibody and vaccination data published by the Office for National Statistics on 23 February 2022, which includes antibody information for England, Wales and Northern Ireland, can be accessed here.
9. 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.
10. ONS have changed from presenting antibody and vaccination data in a fortnightly article to a fortnightly bulletin series, with the last fortnightly article being published on 13 May 2021. Previous articles presenting antibody and vaccination data are still available
11. 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.
12. 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.
13. Previously, published estimates of antibody positivity were 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.
14. 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.
15. The sampling method for Northern Ireland is different to the other nations, inviting only people that have previously participated in a Northern Ireland Statistics and Research Agency (NISRA) survey, which could result in a sample of individuals that are more likely to get vaccinated. This should be taken into consideration if comparing vaccine and antibody estimates across the four nations, since vaccine status and antibody positivity are related.
16. The weekly modelled estimates use standard calendar weeks starting on a 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 week, depending on the availability of test results.
17. Units of measurement: The test used for spike antibodies measures their concentration in ng/ml. The previously reported standard antibody threshold of 42 ng/ml corresponds to 23 binding antibody units (BAU)/ml using the World Health Organisation’s standardised units (enabling comparison across different antibody assays). The higher antibody threshold of 179 ng/ml corresponds to 100 BAU/ml.
18. The higher antibody threshold of 179 ng/ml was determined from analysis during the period when most COVID-19 infections were with the Delta variant. It is likely that the equivalent level of protection for the Omicron variant will require a different antibody threshold.