Merck (NYSE:MRK) released new data today demonstrating that its recently approved monoclonal antibody, Enflonsia (clesrovimab), remains safe and effective for high-risk children entering their second respiratory syncytial virus (RSV) season.
The results from the Phase 3 SMART trial were presented at the 9th Respiratory Syncytial Virus Foundation (ReSViNET) conference in Rome.
The study focused on a particularly vulnerable cohort: toddlers under age 2 with chronic lung disease or congenital heart disease.
These children often face a "double threat" of severe illness during both their first and second years of life.
According to the findings, children who received a second dose of Enflonsia at the start of their second season showed a safety profile consistent with that seen in infants during their first season.
Crucially, the serum concentrations of the antibody—a key secondary endpoint—matched the high levels achieved in healthy infants in the pivotal CLEVER trial.
This data allows researchers to extrapolate the drug's high efficacy rates to this older, high-risk group.
Enflonsia received FDA approval in June 2025 for use in infants entering their first RSV season.
Unlike existing treatments that require monthly injections or weight-based adjustments, Enflonsia is designed as a single-dose injection that provides five months of protection regardless of the child's weight.
Merck confirmed it will share these second-season results with the FDA and other global regulators to seek an expanded indication.
If approved, Enflonsia would become a primary competitor to Sanofi and AstraZeneca’s Beyfortus in the race to dominate the multi-billion dollar RSV prevention market.