RSV wasn’t as hard on U.S. babies last winter. This may be why

Last winter’s respiratory syncytial virus, or RSV, season wasn’t as brutal for U.S. babies. A new study suggests two preventive tools — a maternal vaccine and a monoclonal antibody for infants — may have helped.

The 2024-25 RSV season was the first in which the vaccine and the monoclonal antibody, which can prevent severe RSV lung infections in babies, were widely available. A study of two hospital surveillance systems found that RSV hospitalization rates last winter for babies up to 7 months old were lower than in two combined RSV seasons prior to the COVID-19 pandemic, researchers report May 8 in Morbidity and Mortality Weekly Report. Those earlier RSV seasons were the most recent that were typical — meaning not altered by the COVID-19 pandemic — without the RSV preventive tools.

“To see a decrease in RSV hospitalization rates is really remarkable.”

Kawsar Talaat
Johns Hopkins University Bloomberg School of Public Health

RSV is the top reason infants are hospitalized in the United States. The virus is particularly hard on infants, says Kawsar Talaat, a vaccine researcher and infectious disease physician at Johns Hopkins University Bloomberg School of Public Health, who was not involved in the new study. “One, they don’t have any preexisting immunity to it to make the disease less severe,” she says. Two, infants’ airways are very small. The mucus and inflammation that hit the lungs during an infection can make it especially difficult to get enough air. It’s “like they’re breathing through a tighter and tighter straw.”

In the winter, “if you go into any pediatric hospital, the wards are full of kids with bronchiolitis,” Talaat says. That’s a lung infection most commonly caused by RSV. “To see a decrease in RSV hospitalization rates is really remarkable.”

The U.S. Centers for Disease Control and Prevention recommends using one of the two preventive tools. The RSV vaccine for pregnant people is a one-time shot given during a specific window in the last trimester and usually administered from September to January. The antibodies transfer to the fetus and can protect the baby for about six months after birth. The monoclonal antibody, called nirsevimab, is a lab-made source of antibodies against RSV that’s protective for at least five months. It’s given to babies up to 7 months old born during RSV season, typically October through March, or entering their first season. Both preventive tools target a protein the virus needs to enter cells.

The new study analyzed data from two surveillance systems that track RSV-associated hospitalizations. The RSV-Associated Hospitalization Surveillance Network includes around 300 hospitals in 13 states. For the two seasons before the COVID-19 pandemic, the RSV hospitalization rate for infants up to 7 months old was 15 per 1,000 children. For the 2024–2025 season, it was 8.5 per 1,000 children, a drop of 43 percent.

The other system, the New Vaccine Surveillance Network, keeps tabs on respiratory illness among children who are hospitalized across seven pediatric medical centers in seven major cities. The RSV hospitalization rate for those up to 7 months old fell 28 percent from around 15 per 1,000 children in 2018–2020 to around 11 per 1,000 children this past winter.

In both datasets, the biggest hospitalization rate drop was seen for the youngest infants, those up to 2 months old, at 52 and 45 percent, respectively.

In the United States, “I think we’ll soon see a day when RSV hospitalizations in young infants will be much rarer than it is today, which is so exciting,” Talaat says. As most RSV hospitalizations and deaths happen in countries where children have less access to health care, she says, it’s also “really important to find ways to have these products available to the most vulnerable children.”