“I’m thrilled about the prospect of this, as well as about the prospect of other R.S.V. vaccines in the pipeline,” said Dr. Miller, who is not an adviser to the agency. “This looks as if it will be the first one coming our way, and it’s a long time coming.”

The vaccine under review Thursday was tested in about 7,300 women after the 24th week of pregnancy. About half received a placebo, and half were given the vaccine as a shot. For the first 90 days after birth, six infants in the vaccination group had a serious case of R.S.V., compared with 33 in the placebo group, translating to an efficacy of nearly 82 percent.

The study, published in The New England Journal of Medicine, showed that for six months after birth, the vaccine was 69 percent effective. In the treatment group, 19 babies fell seriously ill compared to 62 in the placebo group.

The main safety concern during the hearing was whether the vaccine was linked to preterm birth, a safety signal that led GSK to halt its trial of a similar R.S.V. vaccine that was being tested in pregnant patients, according to Dr. Hal Barron, a former company executive. The F.D.A. approved that vaccine, called Arexvy, for older adults earlier this month. (Like GSK, Pfizer tested the same vaccine formula in older adults and infants.)

“We quickly halted the trial based on it confirming that the signal was real,” Dr. Barron said in a March 2022 presentation to investors, “but we are still puzzled as to exactly why this occurred.”

The label for the GSK vaccine says that in tests of pregnant women, 6.8 percent receiving the treatment had preterm births, compared to 5 percent in the placebo group.

In the Pfizer study, premature delivery was reported in 5.6 percent of the pregnancies in the treatment group, compared with 4.7 percent in the placebo group. Officials at the F.D.A. reported that the difference was not statistically significant.