“You can see the hesitancy in all this,” said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a member of the F.D.A.’s vaccine advisory committee. “It’s because in our hearts, I think people don’t quite agree with this notion of a booster dose.”

“The door just got bigger and bigger and bigger, it got wider and wider with each step,” Dr. Offit added. “The companies got what they wanted, the administration got what they wanted.”

In interviews, the experts bemoaned the limited data on the safety and efficacy of the booster shots. The data supporting extra doses of the Moderna and Johnson & Johnson vaccines was “of very low quality,” Dr. Kathleen Dooling, a C.D.C. scientist, acknowledged at the committee meeting on Thursday.

Still, some said they felt they had to vote in favor of booster shots of the Moderna and Johnson & Johnson vaccines because they had already recommended boosters of the Pfizer-BioNTech vaccine and did not want to deny other Americans.