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 but who have not been jabbed.
The findings have been published by the Office for National Statistics (ONS) and are based on a sample of adults who had received the vaccine up to May 31.
Want more news and analysis on the virus? Listen to our latest podcast:
They suggest the risk of infection 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 vaccinated, 1,477 (0.5%) were subsequently found to have a new positive infection of Covid-19.
There was a very slight difference between whether the person had received the Pfizer/BioNTech vaccine (0.8% of the total) and Oxford/AstraZeneca (0.3%).
From a sample of 210,918 adults who had received both doses of vaccine, just 0.1% were subsequently found to have a new positive infection.
Possible explanations for infection shortly after getting the vaccine include someone catching Covid-19 before they had received a jab, or exposure to Covid-19 at a vaccination centre, the ONS said.
The analysis comes as separate figures suggest cases of Covid-19 are “rising exponentially” across England, driven by younger and mostly unvaccinated age groups.
A study commissioned by the Government found infections increased 50% between May 3 and June 7, coinciding with the rise of the Delta coronavirus variant which was first detected in India and is now dominant in the UK.
Data from nearly 110,000 swab tests carried out across England between May 20 and June 7 suggests Covid-19 cases are doubling every 11 days, with the highest prevalence in north-west England and one in 670 people infected.
Coronavirus restrictions in England have now been formally extended until July 19, following a vote in the House of Commons on Wednesday.
MPs voted 489 to 60 to approve regulations delaying the easing of the measures, though Prime Minister Boris Johnson suffered a rebellion from members of his own party over the delay.
Professor Stephen Reicher, a member of the Spi-B group of behavioural scientists advising ministers, rejected a suggestion by LBC radio on Thursday that scientists are “delighted” they have “won the argument” over the four-week delay.
He told LBC: “All of us were distraught. I personally was distraught, none of us want those restrictions.
“And we shouldn’t have been in this position if we’d acted properly and if we’d had proper safeguards to our borders, if we’d had proper support for people to self-isolate.”
Prof Reicher said the Delta variant is “more transmissible in houses”, adding: “If you are sick, then to self-isolate in the home is almost impossible if you live in more crowded homes, so we also need to look at accommodation, we need to look at other forms of support.”