Researchers have conducted a multi-site study in and outside the United States to assess the effectiveness of clesrovimab, a monoclonal antibody, in preventing respiratory syncytial virus (RSV) infections that require medical attention and hospitalization among healthy infants. The findings were published in the New England Journal of Medicine on October 2, 2025.
RSV is responsible for an estimated 12.9 million lower respiratory tract infections and 2.2 million hospitalizations globally each year. Most hospitalizations occur among healthy children within their first six months.
Existing preventive options include another monoclonal antibody, nirsevimab, and a maternal bivalent prefusion F vaccine (ABRYSVO). Clesrovimab targets the RSV fusion protein but binds to a site that is present in both prefusion and postfusion conformations. This targeted site is highly conserved, which may lower the risk of the virus developing resistance.
The clinical trial involved 3,614 infants: 2,412 received a single 105-milligram dose of clesrovimab, while 1,202 received a placebo. Participants were monitored for 150 days following injection. Results showed that clesrovimab was 60.4% effective at preventing medically attended lower respiratory tract infections and reduced hospitalizations by 84.2%. The incidence of serious adverse events was similar between those who received clesrovimab and those who received placebo.
On June 9, 2025, the U.S. Food and Drug Administration licensed clesrovimab as a single-dose intramuscular injection for use in infants up to eight months old before or during their first RSV season.
“Contributed by: Paul A. Offit, MD”
