Merck Seeks Expanded Approval for Enflonsia Vaccine in High-Risk Children

Merck is seeking to expand its approval for Enflonsia (clesrovimab-cfor) to include protection against respiratory syncytial virus (RSV) lower respiratory tract disease in high-risk children under two years of age during their second RSV season.

The company has submitted a supplemental Biologics License Application (sBLA) to the US FDA, which has accepted the application and set a target action date of March 22, 2027. The European Medicines Agency (EMA) also accepted an application in July 2026 to expand marketing authorization for the same pediatric population across the EU.

The approval of Enflonsia was initially granted in June 2025 by the US FDA and April 2026 by the European Commission for the prevention of RSV LRTD in infants born during or entering their first RSV season. The preventative monoclonal antibody is administered to infants without requiring weight-based dosing.

The supplemental application relies on data generated from the phase 3 SMART trial, which evaluated the safety, efficacy, and pharmacokinetics of Enflonsia over two consecutive RSV seasons. If approved, vulnerable young children diagnosed with chronic lung disease (CLD), congenital heart disease (CHD), or qualifying preterm birth conditions would be eligible to receive a second-season dose.