COVID-19 Schools Infection Survey Round 4, England: antibody data, March 2021

• Round 4 of the study took place from 15 to 31 March 2021 shortly after schools in England reopened following the third coronavirus (COVID-19) lockdown; pupil antibody results are only available up to Round 2 (which took place in December 2020).

• In March 2021 (Round 4), 21.52% of primary school staff (95% confidence intervals: 17.54% to 25.94%) and 18.66% of secondary school staff (95% confidence intervals: 16.47% to 21.00%) tested positive to SARS-CoV-2 antibodies.

• The seroconversion rates (converting from negative antibody test to positive) between Round 1 (November) and Round 2 (December), and between Rounds 2 and 4 were similar for secondary school staff (5.4 and 5.0 respectively per 1,000 person-weeks).

• For primary school staff, although the seroconversion rate was higher between rounds 2 and 4 compared with the rate seen between Rounds 1 and 2 (7.8 and 4.5 respectively per 1,000 person-weeks), this difference was not statistically significant.

• In December 2020 (Round 2), 13.45% of secondary school pupils (95% confidence interval: 11.67% to 15.40%) tested positive to SARS-CoV-2 antibodies, this was significantly higher than primary school pupils at 9.05% (95% confidence interval: 7.33% to 11.00%); however, there was no significant difference in the seroconversion rate between primary and secondary pupils between November (Round 1) and December (Round 2).

Figure 1 shows the percentage of staff testing positive for SARS-CoV-2 antibodies in Round 4 (15 to 31 March 2021); 21.52% of primary school staff tested positive (95% confidence intervals: 17.54% to 25.94%) compared with 18.66% of secondary school staff (95% confidence intervals: 16.47% to 21.00%).

The antibody test used in this study detects antibodies produced following natural infection, antibodies resulting from coronavirus (COVID-19) vaccination will not be detected. This is different to the antibody test used in the Office for National Statistics (ONS) COVID-19 Infection Survey (CIS) household survey, which will detect antibodies from vaccination as well as infection. The figures reported in this bulletin are therefore lower than those estimated for the overall household population in the CIS survey.

For comparison between rounds, we have used data from the 11 local authorities with at least two primary and two secondary schools participating in all testing rounds. These are not necessarily the same schools or participants tested between the rounds.

The percentage of staff testing positive for SARS-CoV-2 antibodies increased significantly for both primary school and secondary school staff between Round 2 (December) and Round 4; this can be seen in Figure 2.

For primary school staff, in Round 2, 14.61% tested positive for antibodies (95% confidence interval: 11.86% to 17.72%); this increased to 24.95% in Round 4 (95% confidence interval: 21.07% to 29.15%). For secondary school staff, in Round 2, 15.72% tested positive for antibodies (95% confidence interval: 14.01% to 17.55%); this increased to 22.72% in Round 4 (95% confidence interval: 20.31% to 25.27%).

Local authority results can be found in the accompanying dataset. Confidence intervals are wide, so all estimates should be interpreted with caution, however, the general trends observed are in line with expectations given infection trends in the wider community.

The point estimate for the percentage testing positive increased in all local authorities between Rounds 2 and 4. Staff in Norfolk and Bournemouth (both local authorities with some of the lowest levels of COVID-19 infection in the community over the course of the pandemic), have statistically significantly lower rates of antibodies in Round 4 compared with staff in Liverpool, Lancashire, and Barking and Dagenham (areas where prevalence has been high at certain points during the course of the pandemic).

Seroconversion rate (unweighted data)

In the case of the coronavirus (COVID-19), antibody seroconversion is the incidence of antibody test results changing from negative to positive and will capture both symptomatic and asymptomatic infections that may have been missed between testing rounds.

To account for the different follow-up times between the rounds (on average the follow-up time between Rounds 1 and 2 was three weeks and between Rounds 2 and 4 was 15 weeks), the seroconversion rate has been calculated and expressed per 1,000 person-weeks. More details on this methodology are available. These seroconversion figures do not control for other factors that may affect the probability of seroconverting, for example, different distributions between local authorities of those sampled and different age distributions.

Round 1 to Round 2

Of the 2,276 staff who tested negative for antibodies in Round 1 and also tested in Round 2, 37 seroconverted from negative to positive. Taking into account the time between the two rounds, this gives a seroconversion rate of 5.1 per 1,000 person-weeks (95% confidence interval: 3.6 to 7.1).

Round 2 to Round 4

Of the 1,775 staff who tested negative for antibodies in Round 2 and also tested in Round 4, 158 staff seroconverted from negative to positive. Taking into account the time period between the testing rounds, this gives a seroconversion rate of 5.9 per 1,000 person-weeks (95% confidence interval: 5.0 to 6.9).

For secondary staff, the seroconversion rates between Rounds 1 and 2 and between Rounds 2 and 4 was similar (5.4 and 5.0 respectively per 1,000 person-weeks). For primary staff the seroconversion rate was higher between Rounds 2 and 4 compared with Rounds 1 and 2, at 7.8 and 4.5 respectively, however, this difference was not statistically significant.

Pupil antibody data are only available for Round 1 (3 to 20 November 2020) and Round 2 (30 November to 11 December 2020). Data for Round 4 pupil antibody tests are not yet available because of the longer time needed to process the tests.

The percentage of pupils testing positive to SARS-CoV-2 did not increase significantly for either primary or secondary pupils between Round 1 (November 2020) and Round 2 (December 2020).

The percentage of secondary school pupils testing positive in Round 2 was significantly higher than primary pupils (13.45% compared with 9.05%).

Local authority results can be found in the accompanying dataset.

Seroconversion rate (unweighted data)

Of the 3,927 pupils who tested negative in Round 1 and tested in Round 2, 97 were positive in Round 2. Taking into account the time between the two rounds, this gives a seroconversion rate of 7.9 per 1,000 person-weeks (95% confidence interval: 6.4 to 9.6).

There was no significant difference between seroconversion rates of primary school pupils (8.4 per 1,000 person-weeks) and secondary pupils (7.5 per 1,000 person-weeks).

The seroconversion rates for pupils are higher than those seen for staff during the same time period. However, caution should be taken when comparing staff and pupils as the method of sampling and testing platform for antibodies differed. The oral fluid test was used to test for antibodies in pupils as it is non-invasive, however, it does have a lower sensitivity than the finger prick blood test used for staff.

Staff vaccination data were obtained by linking to the National Immunisation Management System (NIMS). Details of the data matching can be found in our methodology article. Of the staff where immunisation status was available, 63.74% had received at least one vaccination before the end of Round 4 (31 March 2021).

Vaccination data by age and local authority results can be found in the accompanying dataset.

As our vaccination rates relate to school staff in 14 local authorities and are not generalisable to all school staff in England the data in this bulletin will differ from the administrative data on vaccinations published by NHS England weekly, which cover all vaccinations given to individuals who have an NHS number and are currently alive in the resident population.

The Coronavirus (COVID-19) Schools Infection Survey analysis was produced by the Office for National Statistics (ONS) in collaboration with our research partners at the London School of Hygiene and Tropical Medicine and Public Health England.

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.

Statistical significance

A result is said to be statistically significant if it is likely not caused by chance or the variable nature of the samples. For more information, see our methodology page on statistical uncertainty.

Data presented in this bulletin are from Round 4 (with comparisons with Round 1 and Round 2) of the COVID-19 Schools Infection Survey (SIS). These findings are from testing for antibodies to SARS-CoV-2 only. Results on current infection can be found in our bulletin published on 4 May 2021.

Reference period

The results presented in this bulletin are from antibody tests conducted in schools in England between 15 and 31 March 2021 – referred to as Round 4 for staff, and antibody results for pupils between 30 November and 11 December 2020 – referred to as Round 2.

Round 3 was due to take place in late January 2021. Testing within schools for this round was cancelled because of schools in England being shut to the majority of pupils during lockdown.

Response rates

As at 16 April 2021, in Round 4 of testing, 4,154 staff participated in at least one current COVID-19 infection or COVID-19 antibody test. This is around 35% of eligible staff in the sampled schools.

Prior to the commencement of Round 4, participation was offered to all year groups in secondary schools (excluding year 11) to improve the sample size; 42 out of the 80 secondary schools that took part in Round 4 testing had extended participation to other year groups. In Round 4 of testing, 11,033 pupils (3,762 primary and 7,271 secondary) participated in at least one current COVID-19 infection or COVID-19 antibody test. The estimated response rate for secondary school pupils, in the year groups that participation was offered to, was 17%. The estimated response rate for primary school pupils was 20%. Details of previous rounds response rates can be found in the accompanying dataset.

Quality

Further quality and methodology information on strengths, limitations, appropriate uses, and how the data were created is available in our methodology article.

Data cleaning and quality assurance is being carried out on data collected as part of the study on an ongoing basis. All estimates presented in this bulletin are provisional results. Estimates may therefore be revised in future publications.

Please refer to the Strengths and limitations section of the COVID-19 Schools Infection Survey, Round 2 bulletin.