Preventing RSV Bronchiolitis in Infants

Kenny Lin, MD, MPH
January 19, 2026

In young infants, bronchiolitis caused by respiratory syncytial virus (RSV) is a common cause of hospitalization, with an annual incidence of 17.9 hospitalizations per 1,000 infants two months or younger in the United States. Although 4 in 5 children hospitalized for RSV have no risk factors, until 2023, prevention strategies for healthy term newborns were limited. That year, the US Food and Drug Administration approved two new options: the monoclonal antibody nirsevimab and maternal vaccination with a bivalent RSV vaccine, both of which are discussed in a STEPS new drug review in American Family Physician. (In June 2025, a second monoclonal antibody, clesrovimab, became available in the United States.)

In a multicenter randomized trial involving more than 8,000 infants, RSV hospitalization occurred in 0.3% of the group that received nirsevimab during the first winter of life compared with 1.5% of the usual care group (NNT = 83). A study of pediatric emergency departments in Spain found that universal RSV prophylaxis with nirsevimab beginning in the 2023-2024 season was associated with decreases in acute bronchiolitis, related hospital admissions, and pediatric intensive care unit admissions of 59%, 63%, and 63%, respectively. Similarly, infants up to 6 months of age whose mothers received RSV vaccine had a lower likelihood of medically attended severe RSV (0.5% vs 1.8% in the control group; NNT = 81). A Cochrane review of six randomized, placebo-controlled trials confirmed the benefits of RSV vaccination during pregnancy, and a retrospective cohort study of more than 54,000 pregnancies found no increases in preterm birth, premature rupture of membranes, or hypertensive disorders of pregnancy.

The American Academy of Pediatrics, American Academy of Family Physicians, and American College of Obstetricians and Gynecologists recommend routine maternal vaccination or monoclonal antibody administration to prevent RSV infections in infants. Two studies at different institutions found that total uptake (vaccine or nirsevimab) was more than 80%, with acceptance of nirsevimab even in individuals who declined other maternal or infant vaccines. In one of the studies, 43% of parents who refused RSV, influenza, and Tdap vaccines during pregnancy consented to nirsevimab for their newborns. As a result, RSV hospitalization rates during the 2024-25 season were 41% to 51% lower than expected rates and rates in prior years, with the largest benefits observed in infants younger than 2 months.

Presented with two equally recommended prevention options, parents may ask whether one is more effective than the other. Although no head-to-head trials have occurred, a population-based cohort study in France found that compared with infants who were protected by maternal vaccination, those who received nirsevimab had lower risks of RSV hospitalization (adjusted hazard ratio = 0.74) and severe respiratory outcomes, including pediatric intensive care unit admission and the need for oxygen therapy and mechanical ventilation. It is unclear how to apply this finding to the United States, where neither prenatal nor well-child care are universally guaranteed. In a pregnant patient between 32 and 36 6/7 weeks gestation from September 1 through January 31 who is willing to receive RSV vaccine, it makes more sense to vaccinate than to wait to give a monoclonal antibody to the newborn.