One shot of Pfizer 90% effective after 21 days: study

A single dose of the Pfizer vaccine provides “very high” protection against COVID-19 after 21 days – without a “booster” dose within the recommended time frame, according to a new study from the University of East Anglia.

The researchers looked at data from Israel, where the vaccine was deployed.

They found that the Pfizer vaccine becomes 90% effective after 21 days, which supports British plans to delay the timing of a second injection.

But they warn that people’s risk of infection has doubled in the first eight days after vaccination – perhaps because people are becoming less careful.

Due to the rapid response nature of this research, it has not yet been peer reviewed.

Principal investigator and COVID-19 expert Professor Paul Hunter, of the Norwich Medical School, UEA, said: “A second dose of Pfizer vaccine would normally be given 21 days or more after the first to supplement 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 thereby reduce the total number of serious infections, hospitalizations and deaths.

“But the move has drawn criticism from some quarters due in part to the belief that a single injection may not confer adequate immunity.

“A recent unpaired pre-print article based on Israel’s experience examined data from 500,000 people who had received the Pfizer vaccine. He reported that a single dose may not provide adequate protection.

“But we saw a number of flaws in the way they looked at the data, including the fact that they did not attempt to estimate the effectiveness of the vaccine from day 18. That would have given a better indication of the effectiveness of a single dose of the vaccine might be the case if the second dose was delayed for up to 12 weeks. “

The research team set out to estimate the effectiveness of the Pfizer vaccine after a single dose – by reanalyzing the actual results from Israel.

They used the data to see how Israel’s vaccination schedule was impacting the number of cases and then estimated the vaccine’s effectiveness over time.

They found that after the initial vaccination, the number of cases increased for eight days before falling back to low levels by day 21.

Professor Hunter said: “Surprisingly, the daily incidence of cases increased sharply after vaccination until about the eighth day, about double. We don’t know why there was this initial increased risk of infection, but it may be related to the fact that people have less protective behaviors as soon as they have the injection.

“We found that the efficacy of the vaccine was still virtually zero until about 14 days after vaccination of people. But after the 14th day, immunity gradually increased day by day to around 90% by day 21 and then did not improve any further. the improvement observed 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 he supports the UK policy of widening the dose gap by showing that a single dose can provide a high level of protection.

“While we don’t know how long this immunity will last beyond 21 days without a second booster, it is unlikely that we will see a major drop in the next nine weeks,” he added.

Professor 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.

“Estimation of the effectiveness of Pfizer COVID-19 BNT162b2 vaccine after a single dose. A reanalysis of a study of real-world vaccination results in Israel” is published on the site medRxiv pre-print server: www.medrxiv.org/content/10.110… 021.02.01.21250957v1