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The Office for National Statistics (ONS) have released a technical article looking at the impact of vaccination on testing positive in the UK.
Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice, Immediate past Chair of the BMA Public Health Medicine Committee, said:
“I am not well-placed to comment on the details of the different statistical methods alluded to; but ONS is highly expert in this area.
“The study involved taking samples and asking people about symptoms, using a sampling frame. Note that it was not dependent on people declaring that they had symptoms, so it will detect people who were infectious but asymptomatic. It also used data from the NHS test and trace system.
“The detection rate will depend to a considerable extent on the sensitivity and specificity of the test used – the now-familiar PCR (or, strictly, RT-PCR: reverse transcriptase polymerase chain reaction) test. As we know, this test is very highly specific – it yields hardly any false positive results; but it lacks specificity, missing up to 30% of true positive results. It will, therefore, underestimate the number of true positives unless this is adjusted for.
“The survey also used antibody tests (requiring a blood sample) on a subset of the sample. Using this additional method provides extra data, using an entirely different method to the PCR testing, which can help validate the PCR test results.
“Using these different data collection methods makes analysis more complicated, but it also has the potential to enrich and increase the value of the data. This is difficult; but if any organisation has the capability to do this well, the ONS does.
“The data categorised people into 7 “exposure groups” – these are entirely appropriate and well-chosen.
The findings are not surprising, but very useful. They confirm that a single dose of vaccine is very similar, in preventing infection (any test positive, including asymptomatic), to natural infection; but after two doses of vaccination people are considerably less likely to be infected. This is important, because it confirms the value of two doses of vaccine against infection (and, we can presume, on onward transmission). It also refutes the claims from some quarters that natural infection is more effective at this than vaccination.
“The data also confirm that the vaccines are better at preventing symptomatic than asymptomatic infection.
“During the delta period, both vaccines were slightly less effective at reducing infection (testing positive); and the Pfizer-BioNTech performed better than the AstraZeneca vaccine. As the authors point out, however, this could be because the AstraZeneca vaccine was used before the Pfizer-BioNTech vaccine, so we should interpret this with caution, as it could be (at least in part) due to waning immunity.
“The study did not discriminate between doses given months previously and those given more recently (at least 2 weeks after the second dose). I have not had time to delve into the details that are included in the data spreadsheet (available via https://www.ons.gov.uk/releases/coronaviruscovid19infectionsurveytechnicalarticleimpactofvaccinationontestingpositiveintheukoctober2021) but on a quick look I could not find any breakdown by age. I note that the regression modelling adjusted for a number of factors, one of which was age, so presumably the data are available.
“This is topical. It is important right now, because of the decision to offer 12-15-year-olds only one dose of vaccine (for the time-being). The data and rationale supporting this this decision have not been made clear. We need to know how likely 12-15-year-olds are to transmit the infection to others, at school; and back into their families and communities, since it is infection in this under-vaccinated group that is currently driving the pandemic in the UK. Unless this age group responds better to vaccination than older age groups (which would not be unprecedented), there will be considerable value in providing them with a second dose, to reduce the spread of the virus via schools.”
All our previous output on this subject can be seen at this weblink:
www.sciencemediacentre.org/tag/covid-19
Declared interests
Dr Peter English: “Dr English is on the editorial board of Vaccines Today: an unpaid, voluntary, position. While he is also a member of the BMA’s Public Health Medicine Committee, this comment is made in a personal capacity. Dr English sometimes receives honoraria for acting as a consultant to various vaccine manufacturers, most recently to Seqirus.”