A single dose of the Pfizer coronavirus vaccine is “highly protective” after three weeks, according to a new study.

Researchers at the University of East Anglia (UEA) said the vaccine developed by Pfizer and BioNTech was still effective after 21 days without a “top up” dose in the recommended time frame.

It comes after a study by the University of Oxford found its coronavirus vaccine with AstraZeneca offers protection of 76% up to 12 weeks after a single dose and may reduce transmission by 67%.

The UEA study, which has not yet been peer-reviewed, looked at data from Israel where the vaccine has been rolled out.

Scientists found the vaccine becomes 90% effective after 21 days – supporting UK plans to delay the timing of a second jab.

While it is not yet known how long immunity lasted beyond 21 days without a second dose, researchers believe it is “unlikely” to majorly decline during the following nine weeks.

But scientists warned people’s risk of infection doubled in the first eight days after the Pfizer vaccine jab, citing people becoming less cautious as a possible cause.

Lead researcher Professor Paul Hunter, from UEA’s Norwich Medical School, said: “A recent non peer-reviewed pre-print paper based on Israel’s experience looked at data from 500,000 people who had been given the Pfizer vaccine. It reported that a single dose may not provide adequate protection.

“But we saw a number of flaws in how they looked at the data including the fact that they did not attempt to estimate the effectiveness of the vaccine from day 18 onwards.

“This would have given a better indication of how effective a single dose of the vaccine could be if the second dose was delayed by up to 12 weeks.”

Researchers analysed data to see the impact of the Israeli vaccination programme on case numbers, and went on to estimate vaccine effectiveness over time.

They found that after the initial vaccination case numbers increased for eight days before declining “to low levels” by day 21.

Prof Hunter said: “Surprisingly, the daily incidence of cases increased strongly after vaccination till about day eight – approximately doubling.

“We don’t know why there was this initial surge in infection risk but it may be related to people being less cautious about maintaining protective behaviours as soon as they have the injection.

“We found that the vaccine effectiveness was still pretty much zero until about 14 days after people were vaccinated.

“But then after day 14 immunity rose gradually day by day to about 90% at day 21 and then didn’t improve any further. All the observed improvement was before any second injection.

“This shows that a single dose of vaccine is highly protective, although it can take up to 21 days to achieve this.

“And it supports the UK policy of extending the gap between doses by showing that a single dose can give a high level of protection.”

He added: “Whilst we do not know how long this immunity will last beyond 21 days without a second booster, we are unlikely to see any major decline during the following nine weeks.”

The Government faced criticism over its decision to delay the timing of the second dose of Covid-19 vaccines until 12 weeks after the first, with some raising concerns over the level of immunity from a single injection.

The University of Oxford announced on Tuesday that its study, which is yet to be peer-reviewed, found vaccine efficacy from two doses is 82.4% with the three-month interval.

Its findings also indicated that those who have been vaccinated are not only protected from the disease but are also not likely to pass on the virus to anyone.