Merck has reported new Phase 3 data evaluating ENFLONSIA™ (clesrovimab) in infants and children under two years of age at increased risk for severe respiratory syncytial virus (RSV) disease. The findings, presented at the 9th Respiratory Syncytial Virus Foundation Conference (RSVVW’26) in Rome, summarize outcomes from the second season of the ongoing Phase 3 SMART trial (MK-1654-007).
The study assessed the safety, efficacy, and pharmacokinetics of ENFLONSIA during a second RSV season in children who remained at high risk due to conditions such as chronic lung disease or congenital heart disease. Merck reported that the second-season safety profile was generally consistent with data from the first season of the trial. Monoclonal antibody serum concentrations observed in the second season were comparable to those measured in healthy infants during the pivotal Phase 2b/3 CLEVER trial (MK-1654-004), supporting potential extrapolation of efficacy to children under two years of age during repeat RSV seasons.
The company stated that it plans to share the new data with the U.S. Food and Drug Administration (FDA) and other regulatory authorities as part of efforts to expand ENFLONSIA’s indication to cover high-risk children through a second RSV season. ENFLONSIA is currently approved in the United States, Canada, and several other markets for use in infants during their first RSV season.
ENFLONSIA (clesrovimab) is a long-acting monoclonal antibody developed to provide passive immunization against RSV for approximately five months, which typically spans one RSV season. The therapy received FDA approval in June 2025 based on data from the CLEVER trial and interim SMART results from the first season.
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