Enflonsia is the first RSV preventive approved for infants that uses a single fixed dose regardless of weight and offers protection throughout a full respiratory syncytial virus season.
The FDA has approved enflonsia (clesrovimab-cfor), a long-acting monoclonal antibody developed by Merck, for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants who are entering their first RSV season. The 105 mg intramuscular dose is designed to provide protection for 5 months—the typical duration of an RSV season.
Approval was based on findings from the phase 2b/3 CLEVER trial, which enrolled infants who were randomized 2:1 to receive enflonsia or placebo. Enflonsia reduced the incidence of medically attended RSV-associated lower respiratory tract infections by 60.5% and RSV-related hospitalizations by 84.3% through 150 days postdose. Efficacy in high-risk infants was extrapolated in the phase 3 SMART trial by comparing pharmacokinetic profiles to those observed in the CLEVER trial. Safety data from both trials were consistent and adverse events were comparable between groups.
The most common adverse reactions included injection-site erythema (3.8%), swelling (2.7%), and rash (2.3%). Enflonsia is contraindicated in infants with a history of hypersensitivity to any of its components. As with other IgG1 monoclonal antibodies, anaphylaxis and other serious hypersensitivity reactions have been observed.
Enflonsia is indicated for passive immunization in full-term and preterm infants who were born during or are entering their first RSV season. For infants undergoing cardiopulmonary bypass surgery during this period, a second 105 mg dose is recommended postoperatively once patients are clinically stable.
Enflonsia may be administered concomitantly with routine childhood vaccinations; no additional safety signals were observed. The CDC’s Advisory Committee on Immunization Practices is expected to issue usage recommendations later this month. Merck anticipates product availability in the U.S. prior to the start of the 2025 to 2026 RSV season.
Additional prescribing and safety information is available via the U.S. prescribing information and medication guide.
Source: Merck & Co, Inc
