Norfolk study finds single Pfizer vaccine shot is 90pc effective without top-up
- Credit: Sarah Lucy Brown
A single dose of the Pfizer Covid vaccine provides “very high” protection without a ‘top up’ dose, according to Norwich researchers.
A University of East Anglia study has found that the vaccine becomes 90pc 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.
Lead researcher and Covid-19 expert professor 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.”
The Norwich research team set out to estimate the effectiveness of the Pfizer vaccine after a single dose – by reanalysing real-world outcomes from Israel, where the vaccine has been rolled out.
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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: “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 90pc 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.”
He added: “While 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.”