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Covid booster shots lose potency after about four months

WASHINGTON - Covid booster shots lose their potency after about four months, the C.D.C. said, raising the possibility that some people may need a fourth dose.

The data released on Friday offer the first real-world evidence of the mRNA shots’ waning power against moderate to severe illness in the United States.

In a striking reversal, federal regulators said on Friday that they would wait for data on whether three doses of Pfizer-BioNTech’s coronavirus vaccine are effective in children younger than 5 before deciding whether to authorize a vaccine for the age group.

The Food and Drug Administration will postpone a meeting of outside experts that was scheduled for Tuesday; the experts were to weigh the evidence and make a recommendation on whether to authorize two doses of the vaccine in young children, as Pfizer had requested.

In a news release, Pfizer-BioNTech said that their three-dose trial for young children was moving briskly, and that the new timetable would allow the F.D.A. to get more data and thoroughly review it. Results are expected in early April.

“Given that the study is advancing at a rapid pace, the companies will wait for the three-dose data as Pfizer and BioNTech continue to believe it may provide a higher level of protection in this age group,” the companies said.

At a news conference, Dr. Peter Marks, who heads the vaccine division of the F.D.A., said that parents would have to wait another two months for a decision while regulators gather and analyze more data. “Yes, some of this was late breaking, but that’s what our job is — to adjust to it,” he said.

Regulators and vaccine manufacturers have been wrestling with how quickly to move to vaccinate roughly 18 million children under 5, the only Americans still ineligible for shots. The highly transmissible Omicron variant is receding in much of the nation, but federal officials have said that nearly 400 children under 5 have died of Covid-19.

In Pfizer’s clinical trial, which tested one-tenth of the adult dosage in the youngest group, its vaccine failed to produce the desired immune response in children ages 2 to 4, producing only 60 percent of the level of antibodies identified for success, according to multiple officials. Children ages six months to two years produced the sought-after level of antibodies. There were no serious safety concerns, officials have said.