FDA Accepts Application to Expand ENFLONSIA's Use in Children Under Two

Merck has announced that the U.S. FDA has accepted its supplemental Biologics License Application (sBLA) for ENFLONSIA, a monoclonal antibody treatment for respiratory syncytial virus (RSV), to include children under two years of age at increased risk for severe RSV disease through their second RSV season.

The application is based on data from the Phase 3 SMART trial, which showed that ENFLONSIA provided significant protection against RSV in high-risk infants during their first and second seasons. The FDA has set a target action date of March 22, 2027, for approval.

ENFLONSIA was approved by the U.S. FDA in June 2025 for use in infants born during or entering their first RSV season, and by the European Commission in April 2026. The treatment is administered via a single dose and provides protection against RSV for up to five months.

Merck's Dr. Macaya Douoguih stated that 'the potentially serious impact of RSV does not stop after their first RSV season' and that the expansion of ENFLONSIA's indication will help protect vulnerable children under two years of age during their second RSV season.