Merck Reports Positive Second Respiratory Syncytial Virus (RSV) Season Data For ENFLONSIA In High-Risk Children, Phase 3 SMART Trial Supports Expanded Use

Merck, known as MSD outside the U.S. and Canada, announced positive second-season results from the Phase 3 SMART trial (MK-1654-007) evaluating ENFLONSIA™ (clesrovimab) in infants and young children at increased risk for severe respiratory syncytial virus (RSV) disease. The findings were presented at RSVVW’26, hosted by the Respiratory Syncytial Virus Foundation in Rome. The trial assessed the safety, efficacy, and pharmacokinetics of the long-acting monoclonal antibody across two RSV seasons.

Data from children under 2 years of age who remained at elevated risk entering their second RSV season showed that the safety profile of ENFLONSIA was consistent with that observed during the first season. In addition, serum concentrations of the monoclonal antibody in these children were comparable to levels seen in healthy infants in the pivotal Phase 2b/3 CLEVER trial (MK-1654-004). These findings support extrapolating efficacy to high-risk children through their second RSV season.

“All children who received ENFLONSIA in their second RSV season were at increased risk for severe RSV disease, and nearly all had chronic lung disease or congenital heart disease,” said Dr. Paolo Manzoni, Head of Maternal–Infant Medicine, University of Torino Hospital Degli Infermi, Ponderano, Italy, and an investigator for the SMART clinical trial. “These new findings from SMART demonstrate the potential of ENFLONSIA to help protect these vulnerable children, who may require an additional dose for their second RSV season.”

Earlier interim results from the first RSV season of the SMART study, combined with data from the CLEVER trial, supported the U.S. Food and Drug Administration's approval of ENFLONSIA in June 2025, followed by additional international approvals. Season 1 data were also presented at IDWeek 2024 and published in the New England Journal of Medicine. The