Merck (NYSE: MRK) announced that the U.S. Food and Drug Administration has accepted a supplemental Biologics License Application for Enflonsia (clesrovimab-cfor) to expand its indication to include prevention of respiratory syncytial virus lower respiratory tract disease in children under two years of age at increased risk for severe RSV disease through their second RSV season.

The FDA has set a Prescription Drug User Fee Act target action date of March 22, 2027.

The European Medicines Agency also accepted an application in July 2026 to expand the marketing authorization for Enflonsia in the European Union for the same patient population.

Both applications are supported by data from the Phase 3 SMART trial (MK-1654-007), a randomized, partially blind, palivizumab-controlled, multicenter study. Full results were published in JAMA Pediatrics in July 2026, with interim results published in the New England Journal of Medicine in September 2025.

The SMART trial enrolled children including those born early or moderately preterm, and those with chronic lung disease of prematurity or congenital heart disease. In the second RSV season, 276 eligible participants received an open-label 210 mg dose of Enflonsia by intramuscular injection.

Enflonsia was first approved by the FDA in June 2025 and by the European Commission in April 2026 for prevention of RSV lower respiratory tract disease in infants during their first RSV season. The drug is currently approved in more than 40 countries for use in infants during their first RSV season.

If the expanded indication is approved, children under two years of age at increased risk — including those with chronic lung disease, congenital heart disease, or early or moderate preterm birth with certain risk conditions — would be eligible to receive an additional dose of Enflonsia for their second RSV season.