WASHINGTON — A first-of-its-kind RSV vaccine for pregnant women guards their newborns against the scary respiratory virus — and federal health advisers on Thursday backed Pfizer’s shot despite some lingering questions.

RSV fills hospitals with wheezing babies each fall and winter, and the virus struck earlier than usual and especially hard in the U.S. this past year.

If the vaccine pans out, “many infants and their parents will breathe easier in the coming years,” said Dr. Jay Portnoy, a member of the Food and Drug Administration advisory panel from Children’s Mercy Hospital in Kansas City, Missouri.

The idea: Give women a single injection late in pregnancy, between 24 weeks and 36 weeks, so they develop RSV-fighting antibodies that pass through the placenta — just like they pass protection against other bugs to their babies.

In Pfizer’s international study of nearly 7,400 pregnant women, maternal vaccination proved 82% effective at preventing severe RSV during babies’ most vulnerable first three months of life. At age 6 months, it still was proving 69% protective against severe illness.

Pfizer said there were no signs of safety problems but the FDA did ask its scientific advisers to consider whether a slight difference in premature birth between vaccinated moms and those given a dummy shot was of concern. Debate over whether that was really a hint of trouble or just due to chance dominated the panel’s daylong meeting.

Pfizer pledged to closely track the vaccine’s real-world use for more evidence. Ultimately the advisers unanimously decided that the shot is effective — and voted 10-4 that there’s adequate safety data. The FDA will consider Thursday’s recommendations in making the final decision on approval.

“If you’re in any sense risking premature births with this vaccine, I think there will be a big price to pay,” said Dr. Paul Offit of the Children’s Hospital of Philadelphia, among the panelists who voted “no” on the safety question.

If the FDA ultimately approves the maternal shot, it would mark a second milestone in the decades-long quest to prevent the respiratory syncytial virus. Earlier this month the FDA approved the world’s first RSV vaccine, rival GSK’s shot for older adults, who also are at high risk. There isn’t a vaccine yet for children, but Pfizer is about to begin testing one.