Merck, known as MSD outside the United States and Canada, reported that the European Commission has approved ENFLONSIA (clesrovimab) for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants during their first RSV season. ENFLONSIA is a long-acting monoclonal antibody given as a single, non–weight-based dose designed to provide protection for about five months, the length of a typical RSV season, and is contraindicated in infants with hypersensitivity to clesrovimab or any excipients. The decision authorizes marketing in all 27 European Union member states as well as Iceland, Liechtenstein and Norway, with timing of country-level availability dependent on national reimbursement and other implementation processes.
The approval is based on results from the pivotal Phase 2b/3 CLEVER trial (MK-1654-004; NCT04767373) in preterm and full-term infants from birth to 1 year of age and interim season 1 data from the Phase 3 SMART trial (MK-1654-007; NCT04938830) comparing ENFLONSIA with palivizumab in infants at increased risk for severe RSV disease. Clinical results from CLEVER and SMART were published in the New England Journal of Medicine in September 2025.
ENFLONSIA is already approved in the United States, Canada, Switzerland and several other markets for use in infants during their first RSV season, and regulatory submissions are ongoing in additional countries worldwide.
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