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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.
Analysis of the data, carried out by researchers at the University of East Anglia with funding from the UK government, flies in the face of an earlier study by Israel which suggested that a dose may not offer protection adequate.
Professor Nachman Ash, in charge of the Israeli vaccination effort, said last month that a single dose seemed “less effective than we had thought,” and was also less than what Pfizer had suggested. Pfizer had said the effectiveness was 52% after a single dose.
But Professor Paul Hunter and Dr Julii Brainard say their reanalysis of the data, which has not been peer reviewed, shows high protection just before the second dose is given at 21 days. However, they warn that the risk of infection doubled in the first eight days after vaccination – possibly because people have become less careful.
The UK has made the decision to administer the second doses of Pfizer / BioNTech and Oxford / AstraZeneca vaccines up to 12 weeks after the first dose in order to maximize the number of people protected.
“But this move has drawn criticism from some quarters due in part to the belief that a single injection may not confer adequate immunity,” said Hunter, of the UEA Medical School in Norwich. .
“A recent unpaired pre-print article based on Israel’s experience examined data from 500,000 people who had received the Pfizer vaccine. He noted that a single dose may not provide adequate protection.
“But we saw a number of shortcomings 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 if the second dose was delayed up to 12 weeks. “
The number of cases in Israel increased among those who received an initial dose, they say, for up to eight days after vaccination, and then declined.
“Surprisingly, the daily incidence of cases increased sharply after vaccination until about day 8 – roughly doubling. We don’t know why there was this initial surge in risk of infection, but it may be related to people being less careful about maintaining protective behaviors as soon as they have the injection, ”said Hunter. .
“We found that the efficacy of the vaccine was still virtually zero until about 14 days after vaccination of people. But after day 14, immunity gradually increased day by day to around 90% by day 21 and then did not improve any further. Any improvement observed was before any second injection.
“This shows that a single dose of the vaccine is very protective, although it can take up to 21 days to achieve this.”
He thinks this should be reassuring for the UK. Researchers aren’t sure how long the protection will last after a single hit, he said, but “we’re unlikely to see a major drop in the next nine weeks.”
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