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expert reaction to latest antibody data from the ONS Infection Survey

The Office for National Statistics (ONS) have released the latest data from their COVID-19 Infection Survey on antibodies.


Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“In many ways the latest fortnightly ONS bulletin on people testing positive for antibodies to SARS-CoV-2, the virus that can cause COVID-19, continues the trends we’ve seen in previous recent bulletins.  The percentage of people in the community populations of all four UK countries who would test positive for antibodies continues to rise.  ONS estimate that, for the latest period that they report on (8-14 March), about half of the community population aged 16+ would test positive for antibodies in England, Wales and Northern Ireland, and about 4 in 10 in Scotland.  People can test positive for antibodies, on the test that ONS use, either if they been infected with the virus at some time during the pandemic, or if they have been vaccinated – the test doesn’t distinguish between the two causes of antibodies.  The upward trend in antibody positivity in the latest data will principally be because more and more people are being vaccinated, since the level of actual virus infections is fairly low across all the UK countries.  The effect of vaccinations also shows up clearly in the figures for the percentage who would test positive for antibodies at different ages.  Rates of testing positive are much the highest in the older age groups, where many people have already received at least one dose of vaccine.  For the latest period in these data (ending 14 March), antibody levels are notably higher in people in the age groups of 65 and older than in younger groups.  That matches what we know has happened with vaccinations, allowing for the fact that antibodies do take a couple of weeks to be detectable in the bloodstream  after someone is vaccinated.  And although antibody positivity is increasing in all age groups, it has been going up fastest in the older age groups.  That’s good news – and the rapid increase in antibody levels will occur in younger age groups too as vaccines roll out across the whole adult population in coming weeks and months.

“A new feature in today’s bulletin is that ONS now include estimates of the percentages of people who have been vaccinated.  These estimates are produced by a rather complicated procedure.  They are based on two different sources of data.  Like the antibody positivity results, and like all findings from the magnificent ONS infection survey, they use data from a reasonably representative sample from the community population (aged 16+ in this case), but they also incorporate data from the administrative system that records which vaccinations have been given.  Putting the data together, and making adjustments to the survey data to allow for any remaining biases in the people who were sampled, does inevitably involve some complicated statistical modelling, and as far as I can tell from the information so far available, this has been done well.  ONS provide estimates of the percentage vaccinated for each UK country, and also by age group and for the English regions.  Overall, on ONS’s estimates, well over half the 16+ community population in England and in Northern Ireland had had at least one vaccine dose by 20 March, just under half in Wales, and about 44% in Scotland.  For the countries other than England, there’s rather a lot of statistical uncertainty about these estimates, compared to the position in England, because fewer people are included in the ONS infection survey in the other countries.  But, even allowing for that, it does appear that the vaccination rate is rather lower in Scotland than in England and in Northern Ireland, though we can’t be sure about the comparison between Scotland and Wales.  ONS do point out, though, that these estimates don’t include people living in communal establishments such as care homes.  Elderly care home residents had top priority for vaccination.  Differences in the way care home vaccinations have been dealt with may account for some of the differences in vaccination rates between countries.  But it still does look to me as if overall vaccination rates have been a bit lower in Scotland, so far, than elsewhere in the UK.  That might be part of the reason for the slightly lower antibody positivity rates in Scotland, but that could also arise partly because fewer people have been infected with the virus in Scotland than elsewhere.  It’s difficult to disentangle these things.

“The ONS report is cautious about the relationship between being positive for antibodies and having immunity to the virus.  I can’t really comment fully on this aspect, since I’m not a virologist or immunologist.  But I’ll draw attention to a couple of points in the report.  It points out that one can have antibody levels that are too low to be detected by the test, but might still help immunity.  It points out that there are other aspects of immunity (for example, involving T cells) that won’t be picked up by antibody tests.  And it points out that it is unclear whether antibody waning means that immunity is decreasing.  (Antibody waning is when a person’s antibody levels fall over time, and perhaps become undetectable by the test.)  One aspect of the data is that, for older age groups in particular, the proportion who would test positive for antibodies is smaller than the proportion who have been vaccinated (though you have to be careful in comparing these figures, because the latest vaccination estimates are for a week later than the latest antibody positivity estimates).  For instance, for the week ending 14 March, in England, well over 99% of people aged 80+ had been vaccinated, but only 86% of that age group were antibody positive.  So some will not be antibody positive, on the test, even though they have been vaccinated.  That may be because their antibody levels are too low to be detected, or because they did have antibody levels but they have waned below the detection limit of the test, or (for a few) it might be because not enough time has elapsed since their vaccination.  None of these mean that the people involved definitely have no immunity (though some of them may be in that position – no vaccine is perfect).  And, as ONS are careful to point out, for some elderly people, the decline in antibody positivity might be because they have not yet had their second vaccine dose, which should give them greater and longer-lasting protection.”



All our previous output on this subject can be seen at this weblink:



Declared interests

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.”


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