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A federal advisory panel voted unanimously Friday to recommend a third shot of Pfizer-BioNTech’s coronavirus vaccine be offered at least six months after the second, but only to people 65 and older and those at high risk for severe COVID-19 regardless of their age.

The committee of scientific experts recommended a far narrower rollout than Pfizer said was needed to address what it described as waning immunity in fully vaccinated people. The company asked the Food and Drug Administration to approve boosters to anyone 16 and older at least six months after their second shot.

But the panel, which operates independently of the FDA, rejected that in a 16-2 vote, saying the New York pharmaceutical giant had provided insufficient evidence that a wider rollout was necessary or that the benefits of a third shot would outweigh the risks of possible side effects.

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The advisory panel weighed in on the contentious issue almost exactly a month after President Biden announced plans to offer boosters, an effort the administration had hoped would start next week.

The FDA, which fully approved the Pfizer vaccine last month, doesn’t have to follow the recommendation of its advisory panels, but it usually does.

Immediately after the vote, Dr. Peter Marks, who is in charge of the FDA biologics center that decides on vaccine approvals, promptly began fine-tuning with the committee who might get a third dose. He said recipients might also include fully vaccinated people who are likely to be exposed to the virus because of their jobs, including health care professions and first responders.

The FDA is expected to make its decision on the panel’s recommendation within a few days. Another independent panel of scientists that advises the US Centers for Disease Control and Prevention is scheduled to meet Wednesday and Thursday to weigh in on who would get boosters and when.

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Dr. Michael Kurilla, a committee member who directs clinical innovation at the National Center for Advancing Translation Sciences in Bethesda, Md., acknowledged there was some data suggesting that protection against the virus waned several months after the second shot. But at this point, he said, that justifies boosters only for those most at risk, chiefly older people and those with underlying conditions such as diabetes and obesity. He and other committee members said the vaccine still appears to provide robust protection against severe illness.

”We need to target the boosters,” he said during a daylong virtual meeting that featured several scientific experts from overseas, including Israel and Britain. Kurilla later added, “We can’t simply be in a position where we will be vaccinating every time we think there’s a problem.”

The Biden administration unveiled plans on Aug. 18 for booster shots to start the week of Sept. 20. Administration officials were alarmed by growing reports of breakthrough infections in fully vaccinated people in the United States and abroad, particularly in Israel, which was the first country to vaccinate a majority of its citizens.

But the president’s announcement angered some federal health regulators and physicians who said he had gotten ahead of the scientific community, which had not reached a consensus on a wide rollout of boosters. Indeed, the consensus that quickly emerged at the committee meeting was that such a move would be premature.

Several members of the Vaccines and Related Biological Products Advisory Committee said that most breakthrough infections in this country are asymptomatic or mild and that cases driven by the highly contagious Delta variant were surging in states with low vaccination rates.

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“It’s clear the unvaccinated are driving transmission in the United States,” said Dr. Amanda Cohn, the CDC’s chief medical officer for vaccine policy and a member of the committee.

Dr. H. Cody Meissner, director of Pediatric Infectious Disease at Tufts Medical Center, endorsed booster shots for older people and those most at risk but said inoculating millions of unvaccinated Americans was far more critical.

“Everybody’s got to get the primary series” of shots, he said. “This booster dose is not going to make a big difference.”

That was also the position of two prominent FDA scientists who were among the coauthors of an article this week in the Lancet. The article in the medical journal argued that there was no credible evidence that the vaccines’ potency against severe disease declined substantially over time.

Biden’s plan to offer booster shots has also drawn criticism abroad. The World Health Organization has urged countries to stop rolling out boosters at least through the end of the year, contending that many poorer countries need more access to vaccines. Experts have said that 11 billion doses are necessary to inoculate 70 percent of the world’s population. Reaching that threshold, they say, would significantly reduce the spread of the virus and lower the risk of new variants emerging.

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The Biden administration insists that the country has enough vaccine supply for booster shots and global donations. Indeed, the administration said Friday that it will buy hundreds of millions more Pfizer doses to donate worldwide, according to a Washington Post story that cited two people familiar with the deal.

Much of the discussion at the advisory committee focused on Israel’s experience with the Pfizer vaccine. Dr. Sharon Alroy-Preis, head of public health services for Israel’s Health Ministry, told the experts that her country’s hospitals would likely have been overwhelmed with severe COVID-19 cases if the nation had not rolled out a third dose in late July to about 2.8 million people.

A Delta-driven surge in cases prompted Israel to begin distributing a booster shot of the messenger RNA vaccine. Without it, Alroy-Preis said, health officials projected that the number of coronavirus hospitalizations would have risen to about 2,000 a day. Instead, there are now roughly 700 or fewer such cases a day.

“If we had not started boosters at the end of July, we would have come to the capacity of Israeli hospitalizations and gone beyond it,” she said. “With the booster dose, we were able to dampen that effect.”

Alroy-Preis and Ron Milo, a professor of systems biology at the Weizmann Institute of Science in Israel, said it’s not clear whether the vaccine’s protection waned because of the Delta variant or because immunity responses generally decline over time.

But a third shot boosted protection against infections and severe cases by roughly tenfold, according to data gathered by Israeli researchers. After boosters, people were about 95 percent protected against infections, roughly the same level they were shortly after having received their second shot.

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But several committee members said that Israel’s experience with COVID and the Pfizer vaccine differed from that of the United States. For one thing, they said, Israel had fully vaccinated more than half of its population by March 25, far sooner than most other countries. As infections fell, venues reopened to the vaccinated, and the prime minister told Israelis to enjoy themselves. By June, all restrictions, including indoor masking, had been jettisoned.

So far, the booster discussion among US health regulators has focused on Pfizer’s vaccine, but Cambridge-based Moderna has also applied to the FDA to distribute a third shot of its messenger RNA vaccine. Johnson & Johnson, the New Jersey drug giant, also says it has evidence that people who received its one-shot vaccine, which uses different technology, could benefit from a booster shot after six months.


Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.