An advisory panel to the Food and Drug Administration voted last week in favor of a vaccine for pregnant women designed to prevent respiratory syncytial virus, or RSV, in infants. If the vaccine is approved by the FDA, it will become the first RSV vaccine to protect infants in the country. A vaccine for older adults received FDA approval in early May.

The news comes just a month after a study published in the New England Journal of Medicine found that pregnant women who received the RSV vaccine had babies who were protected from serious complications of the virus. The study gave 3,682 women the vaccine and 3,676 a placebo. The researchers found that six babies born to women in the vaccine group had a severe lower respiratory tract illness within 90 days after they were born compared to 33 infants in the placebo group, indicating that the vaccine had a nearly 82% efficacy rate against severe RSV.

Efficacy waned over time: Six months after birth, the vaccine was 69% effective.

There were no major safety concerns with the vaccine, although 5.6% of pregnancies in the vaccine group ended in premature delivery compared to 4.7% in the placebo group. (Advisory committee members said the difference is not statistically significant — meaning it's not big enough that it's clear that the vaccine caused the difference in numbers.)

The decision on the vaccine now goes to the FDA, which is expected to make a ruling by August.

Why is the vaccine important?

Doctors say they're encouraged by the news, especially after the rough RSV season the country faced this past winter.

"It's critically important to have this vaccine in the pediatric age group," Dr. Thomas Russo, a professor and chief of infectious diseases at the University at Buffalo in New York, tells Yahoo Life. "The zero to 6-month age group — along with older adults — are at the greatest risk for severe disease and serious outcomes from RSV."

RSV is the most common reason for infant hospitalizations in the U.S. and a "huge driver of pneumonia," Dr. Mike Tsimis, an ob-gyn and maternal-fetal medicine physician at Corewell Health, tells Yahoo Life. In children under 5, RSV leads to 2.1 million outpatient visits, up to 80,000 hospitalizations and up to 300 deaths in the U.S. each year, according to data from the Centers for Disease Control and Prevention. "To date, there is no childhood vaccine for RSV," Tsimis points out.

How does the RSV vaccine work?

The reason the shot is given to pregnant women and not the babies themselves is because the newborn immune system is so immature, Russo explains. "The idea is that we'll vaccinate pregnant women, and they'll make the antibodies that will cross the placenta and offer the baby protection for the first few months of life," Dr. William Schaffner, an infectious disease specialist and professor of medicine at the Vanderbilt University School of Medicine, tells Yahoo Life.

The vaccine is given as a single dose during pregnancy and is intended to help protect the baby for the first six months after birth, Russo says.

Experts say they expect the FDA will approve the vaccine. "The one fly in the ointment is the numerical imbalance of premature birth in the vaccinated group," Russo says. "It wasn't statistically significant, but often we don't know if this was by chance or not in groups this small."

Schaffner says that an RSV vaccine for pregnant women may be available as early as the fall. "We have one for adults aged 60 and older. It may well be that we'll have an RSV vaccine for pregnant women also — that would be very exciting," he says.

He adds that an RSV vaccine for pregnant women is definitely needed. "RSV is the last of the important seasonal viruses for which we don't have a vaccine," he says. "We have one for flu and COVID, and RSV is one that causes an awful lot of illness."

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