One dose of the Pfizer / BioNTech vaccine gives people about 90% protection against Covid in 21 days, according to an analysis of Israel’s mass vaccination program.
The analysis of the data, carried out by researchers at the University of East Anglia with funding from the UK government, goes against an earlier study from Israel, which suggested that a dose may not provide adequate protection.
Professor Nachman Ash, in charge of the Israeli vaccination effort, said last month that a single dose seemed “less effective than we thought” and was also lower than Pfizer had suggested. Pfizer said the effectiveness was 52% after a single dose.
But Professor Paul Hunter and Dr. Julii Brainard say their re-analysis of the data, which has not been peer-reviewed, shows high protection just before the second dose is administered at 21 days. However, they caution that the risk of infection has doubled in the first eight days after vaccination – possibly because people have become less cautious.
The United Kingdom has decided to administer the second doses of the Pfizer / BioNTech and Oxford / AstraZeneca vaccines within 12 weeks after the first dose to maximize the number of people protected.
“But this decision has drawn criticism from some sectors due in part to the belief that a single injection may not provide adequate immunity,” said Hunter, from the UEA medical school in Norwich.
“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 shortcomings in the way they analyzed the data, including the fact that they did not try to estimate the vaccine’s effectiveness as of the 18th of the day. second dose was postponed for up to 12 weeks. “
The number of cases in Israel increased among those who received an initial dose, they say, up to eight days after vaccination, and then decreased.
“Surprisingly, the daily incidence of cases increased sharply after vaccination until about day eight – approximately double. We don’t know why there was this initial increase in the risk of infection, but it could be related to the fact that people are less cautious about maintaining protective behavior as soon as they receive the injection, ”said Hunter.
“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 around 90% on day 21 and has not improved further. All 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.”
He believes this should be comforting for the UK. The researchers do not know how long protection will last after a single shot, he said, but “we are unlikely to see any major decline over the next nine weeks”.