Merck seeks FDA sBLA to extend ENFLONSIA protection for high‑risk under‑2s through second RSV season
- FDA accepted a supplemental BLA to expand ENFLONSIA to children under two at increased risk for severe RSV with a PDUFA date of March 22, 2027.
- EMA accepted a parallel application in July 2026 to expand EU marketing authorization for the same population.
- Applications are supported by Phase 3 SMART trial data presented at RSVVW’26 and published (NEJM interim Sept 2025; JAMA Pediatrics full results July 2026).
- In season 1 ENFLONSIA was given as 105 mg vs monthly palivizumab; season 2 eligible children received a 210 mg open‑label dose (n=276), with common AEs including injection‑site erythema (3.8%).
Regulatory status
The U.S. FDA has accepted a supplemental Biologics License Application to expand ENFLONSIA (clesrovimab‑cfor) for prevention of RSV lower respiratory tract disease in children under two years at increased risk for severe RSV through their second RSV season, with a PDUFA target action date of March 22, 2027. The European Medicines Agency accepted a parallel application in July 2026 to expand EU marketing authorization for the same population.
Clinical evidence
Both filings are supported by data from the Phase 3 SMART trial (MK‑1654‑007), presented at RSVVW’26 in February 2026. Interim season‑1 data were published in the New England Journal of Medicine (September 2025) and full results appeared in JAMA Pediatrics (July 2026).
Trial design and key results
SMART was a randomized, partially‑blind, palivizumab‑controlled multicenter study in infants at increased risk for severe RSV (early or moderate preterm, chronic lung disease of prematurity, or congenital heart disease). In season 1 participants received either a single 105 mg IM dose of ENFLONSIA (n=502) or monthly palivizumab (n=501). In season 2, eligible children received an open‑label 210 mg IM dose (n=276; 138 had received ENFLONSIA and 138 palivizumab in season 1). Nearly all season‑2 recipients (99%) had CLD (n=229) or CHD (n=43).
Product profile and safety
ENFLONSIA is an extended‑half‑life monoclonal antibody administered as a fixed 105 mg/0.7 mL dose regardless of weight and designed to provide protection for about five months. For the proposed expansion, an additional 210 mg dose was used in season 2. Contraindications include prior serious hypersensitivity to components; common adverse reactions were injection‑site erythema (3.8%), injection‑site swelling (2.7%) and rash (2.3%). ENFLONSIA can interfere with some rapid antigen RSV assays; RT‑PCR confirmation is recommended when clinical suspicion persists.
Source: Merck
