FDA has agreed to review a supplemental application for Merck’s RSV preventive antibody, Enflonsia, that would extend coverage to vulnerable children approaching their second respiratory syncytial virus season. A decision deadline now sits at March 22, 2027.
The move signals rising recognition that severe RSV risk does not vanish after infancy. Children born with chronic lung disease, congenital heart defects, or certain preterm complications face renewed danger when the virus returns the following year.
Currently, Enflonsia carries approval in over 40 countries for protecting infants during their first RSV season. US regulators cleared it in June 2025, and European authorities followed in April 2026. The antibody stands alone as an RSV preventive that requires no weight-based dose adjustment.
The new filing seeks clearance for a second-season dose targeting children under age 2 who remain medically fragile. If approved, those diagnosed with chronic lung disease, congenital heart disease, or qualifying prematurity conditions could receive additional protection.
Supporting evidence comes from the phase 3 SMART trial, a randomized, partially blinded study comparing Enflonsia against palivizumab across two consecutive RSV seasons. First-season interim findings appeared in the New England Journal of Medicine in September 2025. Complete results landed in JAMA Pediatrics in July 2026.
Across the first season, researchers randomized 1,003 infants evenly between a single 105 mg injection of Enflonsia and monthly palivizumab doses. The second season saw 276 eligible children receive an open-label 210 mg dose of Enflonsia. Nearly all had underlying chronic lung disease or congenital heart disease.
Dr. Macaya Douoguih, who heads global clinical development at Merck Research Laboratories, framed the filings as a chance to ease RSV’s burden on families and health systems alike.
The European Medicines Agency accepted a parallel application in July 2026. Regulatory momentum continues building, with additional international filings already underway.
