Risk of covid-19 infection plummets 21 days after a vaccination

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A person receives a dose of coronavirus vaccine at a vaccination centre in Harrow, UK

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The chance of getting covid-19 after being vaccinated drops sharply 21 days following a first dose, new analysis suggests.

People who become infected post-vaccination are also less likely to have symptoms than those who test positive for the virus and haven’t been jabbed.

The findings, released by the UK Office for National Statistics (ONS), are based on a sample of adults who had received a coronavirus vaccine up to 31 May.

They suggest that the risk of infection initially increases following a first dose, peaking at around 16 days.

There is then a strong decrease in risk up to around one month after the first dose, and the risk then declines slowly but steadily.

Rates of infection post-vaccination are likely to be very low, however.

Out of a sample of 297,493 people who had been vaccinated, 0.5 per cent were subsequently found to have a new infection of covid-19.

Among those who received the Pfizer/BioNTech vaccine, 0.8 per cent later became infected, compared with 0.3 per cent of those who received the Oxford/AstraZeneca vaccine.

From a sample of 210,918 adults who had received both doses of vaccine, just 0.1 per cent were subsequently found to have a new infection.

Some positive covid-19 tests in people who had recently been vaccinated could be explained by them having been infected unknowingly just before having the jab, the ONS said, or being exposed at a vaccination centre.

The analysis comes as separate figures suggest cases of covid-19 are rising exponentially across England, driven by younger and other mostly unvaccinated age groups.

The results of nearly 110,000 swab tests carried out across England between 20 May and 7 June suggest covid-19 infections are doubling every 11 days, with around 1 in 670 people infected. The highest prevalence is in north-west England.

“Prevalence is increasing exponentially and it is being driven by younger ages,” says study author Steven Riley at Imperial College London.

“Clearly that is bad news… but the key thing to point out here is that we are in a very different part of the epidemic in the UK and it is very difficult to predict the duration of the exponential phase.”

Co-author Paul Elliott, also at Imperial, says: “I think we can take quite a lot of comfort from the fact that when we look in the details, it does appear that there is very, very good protection in the older ages, where there is virtually everyone double vaccinated. And in the younger group under the age of 65, where a much smaller proportion have been vaccinated or double vaccinated, most infections are occurring in the unvaccinated group.”

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