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U.S. FDA Accepts sBLA for ENFLONSIA™ (clesrovimab-cfor) to Update its Respiratory Syncytial Virus (RSV) Lower Respiratory Tract Disease Indication to Include Children Under Two Years at Increased Risk for Severe RSV for Their Second Season

MRK
  • Merck announced that the U.S. FDA accepted a supplemental Biologics License Application (sBLA) to expand ENFLONSIA approval for children under two years at high risk of severe RSV disease through their second RSV season.
  • The FDA set a Prescription Drug User Fee Act (PDUFA) target action date of March 22, 2027, for the U.S. application.
  • The European Medicines Agency (EMA) accepted a similar application in July 2026 to expand marketing authorization in the EU for the same population.
  • If approved, eligible children under two years old will receive an additional dose of ENFLONSIA for their second RSV season.
  • Current ENFLONSIA indications cover infants during their first RSV season, following U.S. FDA approval in June 2025 & European Commission approval in April 2026.
  • Both regulatory filings are supported by data from the Phase 3 SMART trial (MK-1654-007), which evaluated safety & efficacy over two RSV seasons.
  • The SMART trial enrolled infants with early or moderate preterm birth, chronic lung disease, or congenital heart disease & compared ENFLONSIA to palivizumab in season 1.
  • In season 2 of the SMART trial, eligible participants received an open-label 210 mg dose of ENFLONSIA.
  • ENFLONSIA is an extended half-life monoclonal antibody administered without weight-based dosing, with a typical protection duration of 5 months.
  • Common adverse reactions observed include injection-site erythema (3.8%), injection-site swelling (2.7%), & rash (2.3%).
  • ENFLONSIA is currently approved in over 40 countries for use in infants during their first RSV season.
  • Regulatory filings for approval in additional countries & for indication expansion are ongoing globally.