ENFLONSIA (clesrovimab) - (MK-1654-007/SMART)

Merck (NYSE: MRK) said the FDA accepted a supplemental BLA for ENFLONSIA (clesrovimab-cfor) to expand its indication to prevention of RSV lower respiratory tract disease in children under 2 at increased risk for severe RSV disease through their second RSV season; the PDUFA date is March 22, 2027. In July 2026, the EMA also accepted the same indication-expansion application.

The filings are supported by the Phase 3 SMART trial (MK-1654-007; NCT04938830), a randomized, partially blind, palivizumab-controlled study in early or moderate preterm infants and infants with chronic lung disease of prematurity or congenital heart disease over two RSV seasons. In season 1, patients received either ENFLONSIA 105 mg (n=502) or monthly palivizumab (n=501). In season 2, eligible children received an open-label 210 mg ENFLONSIA dose (n=276); 99% had chronic lung disease (n=229) or congenital heart disease (n=43).

Merck said SMART data were presented at RSVVW’26 in February 2026 and published in NEJM in September 2025 (interim season 1 results) and JAMA Pediatrics in July 2026 (full results). ENFLONSIA is already approved in the U.S. and EU for infants entering their first RSV season.