The Office for National Statistics (ONS), London School of Hygiene & Tropical Medicine (LSHTM), and Public Health England (PHE) have released the fourth round of results for their COVID-19 Schools Infection Survey, providing initial estimates of staff and pupils testing positive for SARS-CoV-2 antibodies.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“The ONS Covid-19 Schools Infection Survey was started last autumn. Originally it was rightly seen as a useful tool for tracking infection levels in schools at times when there were important questions about which pupils should be going to school and what precautions should be in place there, at times of high infection. Now, though, with the success of the vaccination programme and other measures in reducing infection levels, the findings aren’t nearly as important as they once were, in my view. Also, these results aren’t very up to date and it’s not at all clear to me how they might possibly affect current policy decisions. There may be some possibility of using them to try to assess the effects of previous interventions and policies in schools, but I think that’s a limited possibility too.
“The data that are new in this release are on antibody levels, and in particular on antibodies in school staff during round four of the survey. The other antibody data on staff, from the first two rounds of the survey last autumn, have already been published. (Round three should have occurred early this year, but was cancelled because schools were closed to most pupils during the lockdown.) But round four took place in the second half of March this year, about two months ago now, and a lot has changed since then. Rates of testing positive for antibodies amongst school staff certainly increased quite considerably between round two (early December) and round four (late March), but that’s not really a surprise. People can develop antibodies either because they are infected with the virus that can cause Covid-19, or because they have been vaccinated. However, the antibody test used in this survey detects only antibodies produced by a natural infection, unlike antibody tests that have been used in some other surveys. There was a major wave of infection between December and March, and some school staff would have been infected during that wave, just as people from every occupational group were infected. The bulletin tells us that, on average in the areas in the survey, in every 1,000 school staff, about 6 changed from testing negative to testing positive for antibodies each week between rounds 2 and 4. But it can’t tell us when this happened, or how that rate compares with figures for other groups of people. Also, there’s quite a lot of statistical uncertainty involved – of the staff tested in both rounds 2 and 4, only 158 changed from testing negative to testing positive for antibodies, and that’s not enough to provide really precise estimates.
“One interesting point, though, is that in late March there were considerable differences between the proportion of school staff in the areas covered that tested positive for antibodies, and the proportion that had been vaccinated. In primary school staff in round 4, 22% tested positive for antibodies arising from infection, but 66% had been vaccinated. In secondary school staff, the figures were 19% testing positive and 59% vaccinated, and as the vaccines roll out further, the percentages of staff who have been vaccinated will be considerably greater now). In terms of immunity from infections, antibodies from an infection and antibodies from vaccination both protect against future infection to a considerable extent (and more so against serious infection or hospitalisation). Because the antibody test used in this survey doesn’t detect antibodies from vaccination, the level of immunity from infection and serious disease in school staff will actually be considerably higher than the antibody levels in the survey indicate. (Also, antibody levels don’t measure all aspects of immunity anyway.)
“The ONS release also includes antibody data on pupils, from the first two rounds, and I believe the antibody data on pupils for round 2, from early December 2020, have not been published before. But those figures are pretty old now, and another issue is that the response rate for pupils was pretty low (16% of pupils) so that there is a lot of uncertainty. Furthermore, pupils who were not at school when the tests were done would not have provided data, and, particularly if they were absent because they had Covid-19 or were self-isolating, that might bias the results. So the pupil antibody data are quite old and not terribly reliable, so I won’t comment further on them.
“A major issue is that the local authorities that were sampled for this survey are not representative of all local authorities in England, and aren’t intended to be. So these results can’t be used to give an estimate of average antibody levels for school staff or pupils across the country. The ONS bulletin makes this very clear. When the local authorities where the survey was to be done were chosen in the early autumn, more local authorities where infection rates were high at the time were chosen, presumably to allow for more accurate surveillance there. This makes it impossible to use the results to provide national estimates – and also, the areas that had high or low prevalence back in September didn’t necessarily have high or low prevalence as the study goes on.”
All our previous output on this subject can be seen at this weblink:
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic. My quote above is in my capacity as an independent professional statistician.”