Independent experts, including government scientists, were skeptical of the research from Israel, which included limited data over a short period.
Wading into an acrimonious debate over booster doses, researchers in Israel reported on Wednesday that a third dose of the Pfizer-BioNTech coronavirus vaccine can prevent both infections and severe illness in adults older than 60 shortly after the injection.
The study, published in the New England Journal of Medicine, is the latest salvo in the conflict over whether booster doses are needed for healthy adults and whether they should be given out, as the Biden administration plans to do, when so much of the world remains unvaccinated.
Several independent scientists said the cumulative data so far suggest that only older adults will need boosters — and maybe not even them.
Vaccination remains powerfully protective against severe illness and hospitalization in the vast majority of people in all of the studies published so far, experts said. But the vaccines do seem less potent against infections in people of all ages, particularly those exposed to the highly contagious Delta variant.
What the Israeli data show is that a booster can enhance protection for a few weeks in older adults — a result that is unsurprising, experts said, and does not indicate long-term benefit.
“What I would predict will happen is that the immune response to that booster will go up, and then it will contract again,” said Marion Pepper, an immunologist at the University of Washington in Seattle. “But is that three- to four-month window what we’re trying to accomplish?”
Federal health officials — including Dr. Anthony S. Fauci, President Biden’s top medical adviser on the pandemic — have justified plans to distribute booster shots by pointing to emerging evidence from Israel and other countries suggesting that immunity from vaccination wanes over time.
The idea has sent some Americans scrambling for booster shots even before they are formally authorized, a step the F.D.A. may take as soon as Friday. But even among government scientists, the idea has been met with skepticism and anger.