European Commission approves Merck’s ENFLONSIA (clesrovimab) for RSV prevention in infants, supported by strong Phase 2b/3 and Phase 3 clinical data.
Written By: Chikkula Pavan Kumar, PharmD
Reviewed By: Pharmacally Editorial Team
The European Commission has approved ENFLONSIA™ (clesrovimab), a long-acting monoclonal antibody developed by Merck, for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants during their first RSV season. The approval covers all European Union member states as well as Iceland, Liechtenstein, and Norway, with country-specific availability subject to local reimbursement processes.
ENFLONSIA is designed as a single-dose preventive option that provides rapid and sustained protection for up to five months, corresponding to a typical RSV season. The therapy uses a fixed, non-weight-based dosing approach, simplifying administration in infants. It is indicated for passive immunization and is contraindicated in those with known hypersensitivity to its components.
The approval is supported by data from the Phase 2b/3 CLEVER trial (NCT04767373) and interim results from the Phase 3 SMART study (NCT04938830). In the CLEVER trial, ENFLONSIA demonstrated a 60.4% reduction in RSV-associated medically attended lower respiratory infections and an 84.2% reduction in RSV-related hospitalizations over five months, with additional reductions observed in severe disease endpoints.
The safety profile was generally comparable to placebo, with most adverse events reported as mild to moderate. Findings from both studies were published in the New England Journal of Medicine in September 2025.
Commenting on the approval, clinical investigator Dr. Paolo Manzoni noted that RSV remains a leading cause of infant hospitalization globally and highlighted ENFLONSIA as a clinically supported preventive option that may help reduce disease burden.
From the company perspective, Dr. Macaya Douoguih of Merck Research Laboratories described the decision as an important step toward expanding access to RSV prevention and addressing an unmet need in infant health.
ENFLONSIA is already approved in several markets, including the United States, Canada, and Switzerland, with additional regulatory submissions ongoing worldwide.
Reference
Efficacy and Safety of Clesrovimab (MK-1654) in Infants (MK-1654-004) (CLEVER), ClinicalTrials.gov ID NCT04767373, https://clinicaltrials.gov/study/NCT04767373
About the Writer
Chikkula Pavan Kumar, PharmD is a Doctor of Pharmacy with a keen interest in clinical pharmacy, pharmacovigilance, and evidence-based practice. In his words, he is passionate about patient safety and translating complex medical information into clear, research-driven communication.