Researchers at the University of East Anglia say the vaccine developed by Pfizer and BioNTech is still effective after 21 days without a "top up" in the recommended time frame.

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

The UEA study, which has not yet been peer-reviewed, looked at data from Israel where the vaccine has been rolled out.

Scientists found it becomes 90 per cent 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 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 they did not attempt to estimate the effectiveness of the vaccine from day 18 onwards," he said.

"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 program on case numbers and went on to estimate vaccine effectiveness over time.

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

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

"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 the vaccine effectiveness was still pretty much zero until about 14 days after people were vaccinated."

Then after day 14 immunity rose gradually day by day to about 90 per cent at day 21 and 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," Prof Hunter said.

Australian Associated Press