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Pfizer vaccine: Single dose ‘90 per cent effective after 21 days' © Getty Images

Pfizer vaccine: Single dose ‘90 per cent effective after 21 days'

Published: 04th February, 2021 at 10:12
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The study, which hasn't yet been peer-reviewed, "supports the UK policy of extending the gap between doses."

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


Researchers at the University of East Anglia (UEA) said the coronavirus 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 found that the Oxford coronavirus vaccine 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The UEA study of the Pfizer jab, which has not yet been peer-reviewed, looked at data from Israel where the vaccine has been rolled out.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 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.

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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.

“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," said lead researcher Professor Paul Hunter, from UEA’s Norwich Medical School.

“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.

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

Graphic showing the number of COVID-19 vaccination doses given in the UK, as of 2 February © PA Graphics
© PA Graphics

“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," he said.

“But then after day 14 immunity rose gradually day by day to about 90 per cent 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.”

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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 2 February that its study, which is yet to be peer-reviewed, found vaccine efficacy from two doses is 82.4 per cent 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.

How do scientists develop vaccines for new viruses?

Vaccines work by fooling our bodies into thinking that we’ve been infected by a virus. Our body mounts an immune response, and builds a memory of that virus which will enable us to fight it in the future.

Viruses and the immune system interact in complex ways, so there are many different approaches to developing an effective vaccine. The two most common types are inactivated vaccines (which use harmless viruses that have been ‘killed’, but which still activate the immune system), and attenuated vaccines (which use live viruses that have been modified so that they trigger an immune response without causing us harm).

A more recent development is recombinant vaccines, which involve genetically engineering a less harmful virus so that it includes a small part of the target virus. Our body launches an immune response to the carrier virus, but also to the target virus.

Over the past few years, this approach has been used to develop a vaccine (called rVSV-ZEBOV) against the Ebola virus. It consists of a vesicular stomatitis animal virus (which causes flu-like symptoms in humans), engineered to have an outer protein of the Zaire strain of Ebola.

Vaccines go through a huge amount of testing to check that they are safe and effective, whether there are any side effects, and what dosage levels are suitable. It usually takes years before a vaccine is commercially available.

Sometimes this is too long, and the new Ebola vaccine is being administered under ‘compassionate use’ terms: it has yet to complete all its formal testing and paperwork, but has been shown to be safe and effective. Something similar may be possible if one of the many groups around the world working on a vaccine for the new strain of coronavirus (SARS-CoV-2) is successful.

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.


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