Pfizer single shot 90 percent effective after 21 days: study

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

The researchers analyzed data from Israel, where the vaccine was launched.

They found that the Pfizer vaccine becomes 90 percent effective after 21 days – supporting the UK’s plans to delay the time for a second injection.

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

Due to the rapid response nature of this survey, it has not yet been peer-reviewed.

Lead researcher and expert in COVID-19, Prof Paul Hunter, from UEA’s Norwich School of Medicine, said: “A second dose of the Pfizer vaccine would normally be administered 21 days or more after the first to complete and prolong the effect of the first dose.

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

“The logic behind this is to protect more people sooner and thus reduce the total number of serious infections, hospitalizations and deaths.

“But this decision has drawn criticism from some sectors due in part to the belief that a single injection may not provide adequate immunity.

“A recent pre-printed, non-peer-reviewed article based on Israel’s experience analyzed 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 analyzed the data, including the fact that they did not try to estimate the vaccine’s effectiveness from the 18th. second dose was postponed for up to 12 weeks. “

The research team began to estimate the effectiveness of the Pfizer vaccine after a single dose – reviewing Israel’s actual results.

They used the data to see how the Israeli vaccination program impacted the number of cases and began to estimate the vaccine’s effectiveness over time.

They found that after the initial vaccination, the number of cases increased for eight days before dropping to low levels on the 21st.

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

“We found that the vaccine’s effectiveness was still practically zero until about 14 days after people were vaccinated. But after day 14, immunity gradually increased, day by day, to about 90 percent on day 21, and then it didn’t improve. more, the improvement observed was before any second injection.

“This shows that a single dose of the vaccine is highly protective, although it can take up to 21 days to achieve this.

“And it supports the UK’s policy of widening the gap between doses, showing that a single dose can provide a high level of protection.

“Although we do not know how long this immunity will last beyond 21 days without a second booster, it is unlikely that we will see any major decline over the next 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.

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