More than 30 million people have gotten additional shots, with the number often outpacing the number of first shots given each day around the country. Booster doses were also authorized in October for everyone who received the single-dose Johnson & Johnson vaccine, and for vulnerable groups who received Moderna’s two-dose shot.
The C.D.C.’s gathering on Friday is scheduled to be briefer than recent meetings about Covid vaccines, just three hours, and is expected to be straightforward, one federal official familiar with the planning said, in part because of how far the nation’s booster campaign has already come. That would suggest a significant softening of opposition among public health experts since President Biden announced in August that he hoped to offer booster doses to all adults.
Mr. Biden wanted to start the campaign in late September, but his announcement was heavily criticized by public health experts, who said he was rushing federal scientists and regulators who had to decide whether the data supported such a move. Members of the F.D.A. and C.D.C. advisory committees have said they were uncomfortable with how quickly the administration wanted to offer booster shots to a large swath of Americans.
Some key regulators and outside advisers had been hesitant to endorse a sweeping booster campaign this early, arguing that effectiveness of the two-dose regimen, especially in preventing hospitalization and death, has held up strongly. One large study in New York of nearly nine million people has shown remarkable durability in the protection gained from all three federally authorized vaccines.
But more recently, federal regulators and scientists have said the evidence is increasingly clear that a reduction in vaccine efficacy against milder Covid cases, and in protective antibodies, has put Americans — especially seniors and those with medical conditions — at risk of harmful breakthrough infections as the Delta variant of the virus only gradually loosens its grip.
“Antibodies decay with time. That’s not just a coronavirus vaccine story,” said Dr. Ofer Levy, director of the Precision Vaccines Program at Harvard’s Boston Children’s Hospital and a member of the F.D.A. advisory committee. “Vaccines vary in the durability of the protective response. We are starting to see waning in other age groups as well.”
An Israeli study published in the scientific journal The Lancet in late October compared about 730,000 people who had received a booster dose in August or September with individuals who had received only two doses of the Pfizer-BioNTech vaccine at least five months earlier, finding that boosted recipients had a lower risk of hospitalization and severe disease. Those with booster shots were evaluated between one week and nearly two months after their third dose.
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