Merck & Co. (MRK) delivered encouraging news Thursday for parents of vulnerable children: its RSV antibody treatment appears to hold up safely through a second respiratory virus season.
The pharmaceutical giant presented findings from the second RSV season of its Phase 3 SMART trial at the RSVVW Conference, showing that Enflonsia (clesrovimab) maintained a safety profile consistent with what researchers observed during the first season. More importantly, the monoclonal antibody achieved serum concentrations comparable to those seen in healthy infants from earlier studies, suggesting the treatment could effectively protect high-risk children who need coverage beyond their initial RSV season.
This matters because Enflonsia currently has FDA approval only for infants during their first RSV season, where it's designed to provide about five months of protection. But some children, particularly those with underlying health conditions or premature births, remain vulnerable in their second year of life. Merck plans to share this new data with the FDA to pursue an expanded indication for these higher-risk kids.
Building on Earlier Success
The original FDA approval in June 2025 came on the strength of interim data from the first RSV season of the SMART trial, combined with results from the pivotal Phase 2b/3 CLEVER trial. Those studies established that the antibody works for standard-risk infants in their first season. Now Merck is making the case that it can do more.
The trial enrolled children under 2 years of age, tracking both safety signals and whether the antibody maintained therapeutic levels in the bloodstream. The consistency across both measures suggests the treatment could become a repeat option for families dealing with conditions like congenital heart disease or chronic lung issues that make RSV particularly dangerous.
Competitive Landscape Heats Up
Merck isn't alone in the RSV prevention space. GSK plc (GSK) presented new data Wednesday for AREXVY, its RSV vaccine targeting older adults. The study showed 75.6% vaccine effectiveness against RSV-related hospitalization at a median follow-up of 5.6 months post-vaccination. Perhaps more striking, exploratory analysis suggested 63.1% effectiveness against major adverse cardiovascular events, an intriguing finding given RSV's potential to stress the heart.
The two approaches target different populations, but both companies are racing to establish their products as the standard of care in their respective markets.
