Coronavirus (COVID-19): ONS Infection Survey – antibody data – 16 March 2021

This data refers to the presence of antibodies to the Coronavirus (COVID-19) within the community population; community in this instance refers to private residential households, and it excludes those in hospitals, care homes and/or other institutional settings. All results are provisional and subject to revision.

This article presents analysis on past infection and/or vaccination, which is defined as testing positive for antibodies to SARS-CoV-2 based on findings from the ONS COVID-19 Infection Survey. SARS-CoV-2 is the scientific name given to the specific virus that causes COVID-19.

ONS present weighted estimates for 28-day periods of antibody positivity for England, Wales, Northern Ireland and Scotland, published fortnightly. The 28-day periods presented in this analysis are updated to work backwards from the most recent 28-day period available. This means the time periods overlap with those presented in previous articles.

Estimated proportion of people in Scotland that would have tested positive for COVID-19 antibodies in the 28 days up to 3 March 2021

In the 28 days up to 3 March 2021, it is estimated that 22.3% of the population in Scotland aged 16 and over would have tested positive for COVID-19 antibodies from a blood sample. A 95% confidence interval for this estimate is 20.6% to 24.1%.

It is estimated that, at any given time in this 28-day period, 996,000 people in Scotland aged 16 and over would have tested positive for COVID-19 antibodies (95% confidence interval: 918,000 to 1,077,000). This equates to around 1 in 4 people aged 16 and over (95% confidence interval: 1 in 5 to 1 in 4).

In Scotland, the estimated percentage of the population aged 16 and over that would have tested positive for COVID-19 antibodies has increased in the most recent 28-day period.

Figure 1: Estimated percentage of the population that would have tested positive for COVID-19 antibodies in 28-day periods from 17 September 2020 to 3 March 2021 in Scotland, including 95% confidence intervals

Estimated proportion of people in Scotland that would have tested positive for COVID-19 antibodies, by age

In the 28 days up to 3 March 2021, it is estimated that the age group with the highest percentage of people testing positive for COVID-19 antibodies from a blood sample was those aged 80 years and over at 60.9% (95% confidence interval: 50.3% to 70.8%). The next highest groups are those aged 75 to 79 years at 39.8% (95% confidence interval 31.7% to 48.2%) and those aged 70 to 74 years at 38.9% (95% confidence interval: 33.1% to 44.9%). The higher levels of antibodies observed in older age groups likely reflects the high vaccination rate in older people.

In those aged under 70, the percentage testing positive for antibodies is lower, ranging from 14.8% to 21.1%.

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

Figure 2: Estimated percentage of the population that would have tested positive for COVID-19 antibodies, by age group, in the 28 days up to 3 March 2021, including 95% confidence intervals

Modelled daily estimates of antibody positivity by single year of age can be found in the accompanying dataset and visualisation published by the ONS.  Figure 3 shows the daily estimate of the percentage of people testing positive for COVID-19 antibodies on 3 February and 3 March 2021, to show the change over time. The model used to produce the figures illustrated in Figure 3 differ from that used in Figure 2, and as such, these figures are not directly comparable. However, the broad pattern across age groups appears similar for the latest data.

Figure 3: Modelled daily percentage of the population aged 16 and over that would have tested positive for COVID-19 antibodies by single year of age on 3 February and 3 March 2021, including 95% confidence intervals

Estimated proportion of people in Scotland that would have tested positive for COVID-19 antibodies, by sex

In Scotland, it is estimated that, in the 28 days up to 3 March, 25.1% of females would have tested positive for COVID-19 antibodies (95% confidence interval: 22.6% to 27.7%). In the same period, it is estimated that 19.3% of males would have tested positive for COVID-19 antibodies (95% confidence interval: 16.9% to 21.9%).

The percentage of people testing positive for antibodies was higher for females than for males in Scotland, England and Wales. Differences between females and males in Northern Ireland are less certain because of a small sample size, leading to wide, overlapping confidence intervals.

In England 37.6% of females would have tested positive for COVID-19 antibodies (95% confidence interval: 36.7% to 38.4%) and 31.6% of males (95% confidence interval: 30.7% to 32.5%). In Wales 35.2% of females would have tested positive for COVID-19 antibodies (95% confidence interval: 30.8% to 39.9%) and 25.5% of males (95% confidence interval: 21.6% to 29.7%).

Figure 4: Estimated percentage of the population that would have tested positive for COVID-19 antibodies, by sex, in the 28 days up to 3 March 2021, including 95% confidence intervals

Estimated proportion of people in Scotland that would have tested positive for COVID-19 antibodies, by sex and age group

In the 28 days leading up to 3 March 2021 the percentage of people testing positive for antibodies was higher for females than males, and this was broadly consistent across the different age groups under age 70.

It is estimated that the percentage of females that would have tested positive for COVID-19 antibodies from a blood sample was higher than for males in the 50-69 age group.

For age groups under 50, it appears that there may be a higher percentage of females who would have tested positive for antibodies than males, although these differences are less certain due to a small sample size, leading to wider, overlapping 95% confidence intervals.

Figure 5: Estimated percentage of the population that would have tested positive for COVID-19 antibodies, by age group and sex in the 28 days up to 3 March 2021, including 95% confidence intervals

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.

These statistics refer to infections reported in the community, by which we mean private households. These figures exclude infections reported in hospitals, care homes and/or other institutional settings. The population used in this analysis relates to the community population aged 16 years and over. It should be emphasised that 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).

Weighted estimates are provided with 95% confidence intervals to indicate the level of uncertainty around them. 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.

The full article on antibody test results published by the Office for National Statistics on 16 March 2021, which includes antibody information for England, Wales and Northern Ireland, can be accessed here.

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.