UK study shows surging efficacy after COVID-19 booster

Jolyon Attwooll

16/11/2021 4:24:40 PM

The country’s first real-world study since its booster program began shows it is having a significant impact on protection for over-50s.

Pfizer syringes and vials.
The report is the first to consider the impact of a booster among recipients whose primary course has been a vaccine other than Pfizer. (Image: AAP)

A newly published study in the UK suggests that a COVID-19 booster dose pushes protection levels even higher than after the completion of a primary vaccination course.
The findings were made by the UK Health Security Agency (UKHSA), which considered the impact of the booster program in those aged 50 and older. This cohort was selected as previous research indicates COVID-19 vaccine efficacy wanes more sharply among older people and those with underlying medical conditions.
The study suggests that two weeks after a booster dose the protection against symptomatic infection in that age group stood at 93.1% for those who had AstraZeneca as their primary vaccination course. It was marginally higher at 94% for those who had previously received the Pfizer vaccine.
Both rates are higher than the protection against infection found after completing the initial two-dose vaccination.
The report is the first to consider the impact of a booster among recipients whose primary course has been a vaccine other than Pfizer. As in Australia, the AstraZeneca vaccine has been widely used in the UK, particularly among older population groups.
COVID-19 vaccine efficacy has been shown to fall against symptomatic infection and there is also emerging evidence regarding a decline in protection against severe disease and death – although studies suggest the overall protection rate remains high.
However, the authors of the UKHSA report say it is too early to assess to what extent the booster is protecting against more severe iterations of COVID-19.
‘Due to small numbers at this early stage of the booster rollout this study only assesses symptomatic disease. There is currently insufficient follow-up to estimate the effects on severe disease which leads to hospitalisation and death,’ the report reads.
‘For the same reason we are only able to report the early effects of the booster program and it is not yet clear how long protection against COVID-19 following booster vaccination will last.’
Booster doses were opened to adults aged 50 and above on 14 September in the UK, and the program has now been extended to adults aged 40 and older.
So far, more than 12.8 million third doses have been administered in the country.
The UKHSA study appears to reinforce other studies indicating the high efficacy of Pfizer against infection when administered as a third dose.
However, while appearing to produce reassuring results, the authors warn that there could be confounding factors at play.
‘This is an observational study with a number of possible biases and should be interpreted with caution,’ they wrote.
‘The imperfect sensitivity [of] PCR testing could cause misclassification of both cases and controls, which could attenuate vaccine effectiveness estimates.’
They also said the prevalence of COVID-19 in the UK could have an impact.
‘Many individuals will also have been previously infected so the VE [vaccine efficacy] measured is in the context of a population where many have already had natural exposure,’ the report reads.
‘We adjust for measured confounders, however, there may be residual confounding that we could not account for.’
Despite the caveats, the study suggests ‘residual confounding is small’.
Much of the earliest data on third doses has emerged from Israel, which was the first country in the world to begin a COVID-19 booster program. Pfizer has been used both for the primary course and boosters in that country.
The rollout of boosters in Australia began this month, with access to a third dose opening up to everyone who completed the initial two-dose course at least six months ago.
Last month, the Federal Government announced it was opening access to a third dose for the severely immunocompromised as part of a primary vaccination course.
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