RAHWAY, N.J., August 6, 2026

Merck announced that the U.S. Food and Drug Administration (FDA) has accepted a supplemental Biologics License Application (sBLA) for ENFLONSIA™ (clesrovimab-cfor) to expand its indication for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease (LRTD) to include children under two years of age who remain at increased risk for severe RSV disease during their second RSV season. The FDA has assigned a Prescription Drug User Fee Act (PDUFA) target action date of March 22, 2027. In parallel, the European Medicines Agency (EMA) has also accepted a marketing authorization variation application for the same expanded indication. If approved, eligible high-risk children—including those with chronic lung disease (CLD), congenital heart disease (CHD), or certain preterm birth-related conditions—would be able to receive an additional dose of ENFLONSIA to extend protection through a second RSV season.

Phase 3 SMART Trial Supports Regulatory Expansion

The regulatory submissions are supported by positive findings from the Phase 3 SMART (MK-1654-007) clinical trial, a randomized, partially blinded, palivizumab-controlled, multicenter study evaluating the safety, efficacy, and pharmacokinetics of ENFLONSIA over two RSV seasons. During the first RSV season, infants received either a single 105 mg dose of ENFLONSIA or monthly palivizumab, while eligible children entering a second RSV season received an open-label 210 mg dose of ENFLONSIA. The study enrolled infants born prematurely as well as children with CLD or CHD, populations known to remain vulnerable to severe RSV disease beyond infancy. Clinical data from the SMART study were presented at RSVVW’26, published in the New England Journal of Medicine and JAMA Pediatrics, providing robust evidence supporting the expanded use of ENFLONSIA in this high-risk pediatric population.

ENFLONSIA Continues Global Expansion

ENFLONSIA was originally approved by the FDA in June 2025 and by the European Commission in April 2026 for preventing RSV lower respiratory tract disease in newborns and infants during their first RSV season. It remains the first and only RSV preventive monoclonal antibody administered without weight-based dosing, delivering rapid and durable protection for approximately five months with a single fixed-dose injection. The product has now received approval in more than 40 countries, including the United States, European Union, Canada, China, and Japan, with additional regulatory filings underway globally. The therapy is designed to simplify RSV prevention by providing consistent dosing while reducing the burden associated with repeated monthly prophylaxis currently required with alternative therapies.

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Merck Strengthens RSV Prevention Strategy

The latest regulatory milestone reinforces Merck’s commitment to expanding RSV prevention across vulnerable pediatric populations, particularly children who continue to face elevated risks despite protection during their first RSV season. RSV remains one of the leading causes of infant hospitalization worldwide, contributing to bronchiolitis, pneumonia, and other serious respiratory complications. By extending ENFLONSIA’s potential indication to include high-risk children during their second RSV season, Merck aims to further reduce hospitalizations, disease burden, and healthcare costs associated with severe RSV infections. With simultaneous regulatory reviews underway in both the United States and Europe, the company continues to strengthen its global pediatric infectious disease portfolio while advancing broader access to innovative long-acting monoclonal antibody therapies for RSV prevention.

Source: Merck press release