US pharmaceutical company Merck & Co Inc (NYSE:MRK), known as MSD outside the US and Canada, on Thursday reported positive second-season results from the Phase 3 SMART trial evaluating Enflonsia (clesrovimab) in infants and children under two years at increased risk for severe respiratory syncytial virus (RSV) disease.

Data presented at the 9th Respiratory Syncytial Virus Foundation conference in Rome, Italy showed that safety in the second RSV season was generally consistent with observations in infants treated during the first season. Serum concentrations in treated children were similar to those observed in healthy infants in the Phase 2b/3 CLEVER trial, supporting extrapolation of efficacy through a second RSV season.

Interim first-season SMART data, together with CLEVER results, supported US FDA approval of Enflonsia in June 2025 and subsequent global approvals for use in infants during their first RSV season. The new findings will be submitted to US and international regulators to support a potential label expansion to high-risk children entering a second RSV season.

Enflonsia is a long-acting preventive monoclonal antibody designed to provide up to five months of protection with a single dose regardless of infant weight. Regulatory filings are ongoing in additional markets.