Merck reported that the Phase 3 SMART trial of its RSV antibody ENFLONSIA (clesrovimab) produced positive safety and efficacy results for the second RSV season in infants and children under two years of age who are at increased risk for severe disease. The trial, which enrolled 502 infants in the first season and 276 children in the second season, showed that safety was consistent with the first‑season data and that serum concentrations achieved in the second season were similar to those observed in healthy infants in the pivotal CLEVER trial. The findings support the extrapolation of efficacy to children under two years of age at increased risk through their second RSV season, providing a strong basis for a potential expanded indication.
The data are a significant clinical milestone for Merck’s RSV portfolio. ENFLONSIA is currently approved by the FDA for the prevention of RSV lower respiratory tract disease in newborns and infants entering their first RSV season. The new second‑season data could broaden the drug’s market to include high‑risk children through their second season, matching the label coverage of Sanofi and AstraZeneca’s Beyfortus, which is already approved for infants up to two years old who remain vulnerable during the second season. The single‑dose regimen of 105 mg, regardless of infant weight, offers a convenience advantage over Beyfortus’s weight‑based dosing options.
Dr. Paolo Manzoni, an investigator for the SMART trial, noted: “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.” Vice‑president Macaya Douoguih added: “These new data from the SMART study further position ENFLONSIA as an important potential new option to help protect young children who remain at high risk entering their second RSV season.” The safety profile was consistent with palivizumab, with no drug‑related serious adverse events reported and common adverse events including irritability, somnolence, and decreased appetite.
The trial’s participant profile was primarily children with chronic lung disease or congenital heart disease, the groups most at risk for severe RSV disease. By demonstrating comparable serum concentrations and a consistent safety profile across two seasons, the data strengthen Merck’s case for regulatory approval of the expanded indication and reinforce the company’s broader vaccine pipeline. The positive second‑season results position ENFLONSIA to compete more effectively with Beyfortus and could expand Merck’s market share in the RSV prevention space.
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