An advisory panel to the Food and Drug Administration voted on Thursday in favor of approving a vaccine by Pfizer to prevent the severe respiratory virus that is a potentially deadly threat to infants.
The vaccine would be the first to protect babies from respiratory syncytial virus, or R.S.V., which is the reason many infants are admitted to children’s hospitals each year and kills several hundred under 5 each year.
Fourteen agency advisers unanimously agreed that the vaccine was effective, and the F.D.A. typically follows the recommendations of its advisory panels.
Ten of the 14 agreed that the vaccine was safe, with some airing concerns about elevated rates — not all statistically significant — of preterm births among mothers who got the vaccine compared to those who received a placebo.
The vote follows the F.D.A.’s earlier decision to approve the first R.S.V. vaccine for older adults in the United States. Several other options are still being evaluated.
The Pfizer vaccine for pregnant women, called Abrysvo, is being reviewed ahead of another option submitted to the F.D.A. that would be given to infants — a monoclonal antibody shot meant to provide five months of protection.
R.S.V. is a common ailment that is most severe in young infants and older adults. According to the Centers for Disease Control and Prevention, up to 80,000 children younger than 5 are hospitalized with the virus each year and up to 300 die. (As many as 160,000 adults 65 and older are hospitalized each year with the virus, and about 10,000 die.)
The youngest infants face the greatest risk. Data presented at the meeting showed that infants 6 months or younger were twice as likely to be hospitalized compared with older babies or children. Efforts to test a vaccine in infants began in the 1960s but were abandoned when the vaccine caused more severe cases, said Dr. Bill Gruber, the head of clinical vaccine research and development at Pfizer.
The prospect of having a large number of babies immunized in the fall, before the winter when R.S.V. rates are typically highest, would be “huge,” said Dr. Jonathan Miller, a pediatrician who sees children in the clinic and hospital for Nemours Children’s Health, Delaware Valley.
“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.
Pfizer said if the drug were approved, the company would conduct a post-approval study of real-world use of the vaccine, monitoring health records for the incidence of preterm birth and other possible problems. Agency advisers, though, expressed skepticism about a plan to use data generated from health care billing records to monitor vaccine safety. Several noted that such data could make it hard to link a parent who got the vaccine to the child.
“I do feel like we should be setting the bar higher for review,” said one adviser, Dr. Amanda Cohn, the director of the division of birth defects and infant disorders at the C.D.C., adding that more data might help clarify questions about the effects on preterm birth.
Dr. Hana El Sahly, the advisory committee chairwoman and professor of virology at Baylor College of Medicine, said the number of preterm births among those given the vaccine in a prior Pfizer study, in the main study under review and in the GSK study of a similar product were concerning, particularly given that the United States is not in the midst of an R.S.V. outbreak. She said the pattern should have been examined more carefully.
“That was a big missed opportunity and I feel it’s unfair that we kicked the can down the road to the larger public,” said Dr. El Sahly, who voted “no” to the question about whether the safety data was adequate.
There is another remedy under regulatory consideration, a monoclonal antibody shot developed by Sanofi and AstraZeneca, called nirsevimab. It is meant to be given at the hospital to babies who are born during the winter or in the fall, Jonathan Heinrichs, a Sanofi executive said in an interview.
The medication is under F.D.A. review and was found in one study of nearly 2,500 infants to reduce cases of severe R.S.V. by 75 percent.