Single Pfizer shot 90 per cent effective after 21 days

Published by  Communications

On 4th Feb 2021

A single dose of the Pfizer vaccine provides “very high” protection from Covid-19 after 21 days - without a ‘top up’ dose in the recommended time frame, according to a new study from the University of East Anglia.

Researchers looked at data from Israel, where the vaccine has been rolled out.

They found that the Pfizer vaccine becomes 90 per cent effective after 21 days – supporting UK plans to delay the timing of a second injection.

But they warn that people’s risk of infection doubled in the first eight days after vaccination – possibly because people become less cautious.

Due to the rapid response nature of this research it has not yet been peer reviewed.

Lead researcher and Covid-19 expert Prof Paul Hunter, from UEA’s Norwich Medical School, said: “A second dose of the Pfizer vaccine would normally be given 21 days or more after the first to top up and lengthen the effect of the first dose.

“But here in the UK, the decision was made to delay the timing of the second injection until 12 weeks after the first.

“The logic behind this is to protect more people sooner and so reduce the total number of severe infections, hospitalisations, and deaths.

“But this decision caused criticism from some quarters due in part to a belief that a single injection may not give adequate immunity.

“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.”

The research team set out to estimate the effectiveness of the Pfizer vaccine after a single dose – by reanalysing real-world outcomes from Israel.

They used the data to see how the Israeli vaccination programme impacted 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 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.

“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,” he added.

Prof Hunter and Dr Brainard were funded by the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Emergency Preparedness and Response at King’s College London in partnership with Public Health England (PHE) in collaboration with the University of East Anglia.

Estimating the effectiveness of the Pfizer COVID-19 BNT162b2 vaccine after a single dose. A reanalysis of a study of ‘real-world’ vaccination outcomes from Israel’ is published on the medRxiv pre-print server.

Study with us

Explore our research

 

Latest News

 
View from an airplane over the Arctic.
11 Aug 2022

Arctic flights to shed light on sea ice and storms link

Scientists from the University of East Anglia are spending the summer flying research aircraft through the heart of Arctic storms.

Read more >
 
A whirlpool of water.
10 Aug 2022

New quantum whirlpools with tetrahedral symmetries discovered in a superfluid

An international collaboration of scientists has created and observed an entirely new class of vortices - the whirling masses of fluid or air.

Read more >
 
Dr Leticia Yulita
09 Aug 2022

Language and intercultural expert awarded highest teaching accolade

Dr Leticia Yulita, who teaches Spanish and Intercultural Communication in the School of PPL, has been awarded a National Teaching Fellowship, the highest...

Read more >
 
The Thames riverbed impacted by drought
03 Aug 2022

Why we could be heading for a drought and what we can do about it

England and Wales have seen the driest start of the year since 1976 and it has raised concerns that the UK could be heading for a drought.

Read more >
Are you searching for something?
 
The Thames riverbed impacted by drought
03 Aug 2022

Why we could be heading for a drought and what we can do about it

England and Wales have seen the driest start of the year since 1976 and it has raised concerns that the UK could be heading for a drought.

Read more >
 
UEA campus
28 Jul 2022

UEA campus wins sixth straight international Green Flag Award

UEA campus has won the internationally renowned Green Flag award for a sixth year in succession. 

Read more >
 
Bluegrass under the sea.
28 Jul 2022

Carbon removal using ‘blue carbon’ habitats “uncertain and unreliable”

Restoring coastal vegetation – so called ‘blue carbon’ habitats – may not be the nature-based climate solution it is claimed to be, according to a new study.

Read more >
 
A woman wearing a plastic face shield.
28 Jul 2022

Face shields don’t give high level Covid protection, study shows

If you wore a face shield during the pandemic, it probably didn’t give you a high level of protection against Covid, according to new research from the...

Read more >
 
An Indian woman uses a washing machine.
22 Jul 2022

Gender pay gap linked to unpaid chores in childhood

Young women and girls' time spent in unpaid household work contributes to the gender pay gap, according to new research from the Universities of East Anglia...

Read more >
 
Prof Andy Jordan
22 Jul 2022

UEA professor receives prestigious British Academy fellowship for a lifetime devoted to climate change

Prof Andy Jordan, Professor of Environmental Policy at the University of East Anglia (UEA), has joined the likes of Dame Mary Beard and Sir Simon Schama in being...

Read more >