Merck ENFLONSIA sBLA Accepted: FDA Will Review RSV Protection for Kids Under 2

The FDA accepted Merck’s (MRK) supplemental Biologics License Application for ENFLONSIA on August 6, seeking to expand the RSV monoclonal antibody’s indication to children under 2 years old at increased risk for severe RSV disease. Merck shares closed at $128.33 on August 5, with a market cap of roughly $325 billion.

ENFLONSIA (clesrovimab-cfor) is already approved for infants during their first RSV season, with FDA authorization in June 2025 and European Commission approval in April 2026. This sBLA extends the label into a smaller but higher-acuity group: children with chronic lung disease, congenital heart disease, or prematurity who remain vulnerable through their second RSV season.

The SMART trial provided the supporting evidence. Antibody concentrations in high-risk children under 2 who received a second-season dose of ENFLONSIA were comparable to levels in healthy infants from the registrational CLEVER trial. The data support a strategy of annual monoclonal antibody dosing through the second RSV season for children who do not outgrow their risk after infancy. Merck has not yet disclosed the PDUFA goal date for the sBLA.

The RSV prevention field has become one of biopharma’s fastest-growing markets. Sanofi and AstraZeneca’s Beyfortus (nirsevimab) was the first infant RSV monoclonal antibody approved and generated over $2 billion in 2025. Merck is building ENFLONSIA into a franchise that reaches from newborns through second-season high-risk children, while also developing an adult RSV vaccine program. The RSV market is increasingly crowded, with GSK, Moderna, and Pfizer all competing in the adult vaccine segment and Sanofi holding the infant antibody lead.

The pediatric RSV antibody market is driven by the 2023-24 CDC universal recommendation for infant RSV protection, which made seasonal RSV immunoprophylaxis standard of care for all infants. The recommendation transformed the market from a niche pediatric subspecialty product to a universal infant health measure, driving the multibillion-dollar commercial opportunity. Expanding into the second RSV season captures the patients who age out of first-season coverage but remain clinically vulnerable, a population that previously had no approved prophylaxis option beyond the older and less convenient Synagis (palivizumab), which required monthly injections throughout the RSV season.

What to watch next: the PDUFA date. Once the FDA sets the review clock, the timeline will determine whether ENFLONSIA’s expanded label is available for the 2026-27 RSV season or slips into 2027. A Priority Review designation would put it on a six-month track; standard review is ten months. Merck has not indicated whether it requested priority review for this filing.