JERUSALEM — A fourth shot of the Pfizer-BioNTech coronavirus vaccine generated a fivefold boost in antibodies a week after the jab, according to preliminary results of a study made public by the Israeli government Tuesday.
Israeli Prime Minister Naftali Bennett, in a statement, said the preliminary results indicated “a very high likelihood that the fourth dose will protect vaccinated people to a great degree against infection to some degree and against severe symptoms.”
Israel this week became the first country to launch a major second-booster campaign, making the fourth vaccination available to anyone 60 and older whose last shot was at least four months ago. Bennett’s office said more than 100,000 Israelis have registered or been vaccinated for their fourth shot in the two days of the campaign.
Critics had said the move was premature in the absence of data on a second booster’s safety and effectiveness. Some researchers didn’t rule out that repeat doses of the same vaccine could dampen the body’s immune response.
But the study, conducted by Israel’s Sheba Medical Center, could steer more policymakers toward an aggressive booster policy if it holds up to further analysis. Previous research from South Africa and Britain have suggested that omicron, while more infectious than other coronavirus variants, is less likely to cause serious illness in fully vaccinated individuals.
Researchers recruited subjects from the first round of Israelis given the second booster shot last month, 150 healthy adult medical workers of all ages. All had received previous doses of the Pfizer vaccine. The study first screened for adverse reactions to the fourth shot but found nothing concerning.
“The fourth shot acted just like the first and second shots,” said Sheba spokesperson Steve Walz. “A few people had low fevers, a few had sore arms but nothing more than that.”
On Monday, a week after the boosters were administered, researchers began testing each person’s level of coronavirus antibodies and found the average fivefold jump. “It’s working,” Walz said. “It seems the fourth vaccine is just as effective.”
But some experts, in Israel and internationally, questioned the wisdom of rushing out another round of shots for those who are fully vaccinated when huge numbers of people have yet to be inoculated at all, either by their own choice or because of shortages.
The head of Britain’s lead advisory body on vaccinations warned Monday that too many booster rounds would divert resources from the broader effort to reach as many people as possible.
“We can’t vaccinate the planet every four to six months,” Andrew Pollard, chairman of Britain’s Joint Committee on Vaccination and Immunization, said in an interview with the Telegraph. “It’s not sustainable or affordable. In the future, we need to target the vulnerable.”
Public health officials in Israel said the new findings added welcome information about the likely effectiveness of a fourth Pfizer shot. But some cautioned that the strategy of offering the additional booster was premature.
Hagai Levine, an epidemiologist at Hebrew University and chairman of the Israeli Association of Public Health Physicians, applauded the government’s decision to offer the booster to those with compromised immune systems. But for healthy individuals, it was still unclear whether the immune effects of the first three Pfizer doses had waned so much that a fourth dose was called for, he said.
“It’s kind of the right answer to the wrong question,” Levine said of the new study results. “That it raises the antibody levels is good news, but first we need to know if there is a need for another shot.”
Later this week, Walz, the Sheba spokesman, will be a volunteer in another study in which recipients of previous Pfizer vaccine doses will be injected with a booster shot of the Moderna vaccine. The research will test a theory that mixing and matching the serums could produce a stronger and/or longer-lasting immune response.