Merck, known as MSD outside the United States and Canada, announced on April 20, 2026, that the European Commission has granted marketing authorization for Enflonsia (clesrovimab) for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants. This authorization covers the use of the long-acting monoclonal antibody during an infant’s first RSV season. Enflonsia distinguishes itself as the first and only RSV preventive option in the European Union that does not require weight-based dosing, utilizing a single fixed-dose administration for all eligible infants regardless of their size.
The European Commission’s decision follows a positive recommendation from the Committee for Medicinal Products for Human Use (CHMP). The approval is primarily supported by data from the Phase 2b/3 MK-1654-004 clinical trial, a double-blind, placebo-controlled study involving healthy preterm and full-term infants. In this trial, clesrovimab met its primary safety and efficacy endpoints, demonstrating a significant reduction in the incidence of medically attended lower respiratory tract infections (MALRTI) caused by RSV compared to placebo. Specifically, the trial data indicated that a single dose of Enflonsia provided sustained protection throughout the typical five-month RSV season.
Dr. Dean Y. Li, president of Merck Research Laboratories, stated that the approval represents a significant advancement in pediatric preventative care. He noted that RSV remains a leading cause of hospitalization for infants in Europe and that a simplified, fixed-dose regimen could reduce the administrative burden on healthcare providers while ensuring broad protection across the infant population. The company highlighted that the fixed-dose approach eliminates the need for weight-calculation or multiple vial sizes, which are common requirements for existing monoclonal antibody treatments like nirsevimab.
RSV is a highly contagious virus that affects the lungs and breathing passages. While it often causes mild, cold-like symptoms, it can lead to severe bronchiolitis and pneumonia in infants. According to European health authorities, RSV-related complications result in thousands of pediatric hospitalizations annually across the member states. The introduction of Enflonsia provides a new tool for public health officials to manage seasonal surges in respiratory illnesses.
With this marketing authorization, Enflonsia is now approved for use in all 27 European Union member states, as well as Iceland, Liechtenstein, and Norway. Merck confirmed that it is working with national health authorities to facilitate the availability of the treatment ahead of the 2026-2027 RSV season. The company also continues to pursue regulatory filings for clesrovimab in other global markets, including the United States. The approval marks a major expansion of Merck’s infectious disease portfolio, specifically within the neonatal and pediatric respiratory health sector.