Enflonsia FDA Approval History
Last updated by Judith Stewart, BPharm on June 9, 2025.
FDA Approved: Yes (First approved June 9, 2025)
Brand name: Enflonsia
Generic name: clesrovimab-cfor
Dosage form: Injection
Company: Merck
Treatment for: RSV Vaccination and Immunization
Enflonsia (clesrovimab-cfor) is a respiratory syncytial virus (RSV) F protein-directed fusion inhibitor indicated for passive immunization for the prevention of RSV lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.
- RSV is a contagious virus that causes widespread seasonal infections and can lead to serious respiratory conditions such as bronchiolitis and pneumonia in infants.
- Enflonsia is a preventive, long-acting monoclonal antibody (mAb) designed to be administered at the same single dose (regardless of weight) to infants for direct, rapid and durable protection against a spectrum of RSV disease severity, including worsening disease requiring hospitalization.
- FDA approval was based on results from the pivotal Phase 2b/3 CLEVER trial (MK-1654-004) trial which met its primary and key secondary endpoints:
- Enflonsia demonstrated a reduction in incidence of RSV-associated medically attended lower respiratory infections (MALRI) requiring ≥1 indicator of lower respiratory infection (LRI) or severity compared to placebo through 5 months (primary endpoint) by 60.5% (95% CI: 44.2, 72.0, p<0.001) (incidence rates: Enflonsia, 0.026; placebo, 0.065).
- Enflonsia demonstrated a reduction in RSV-associated hospitalizations through 5 months (key secondary endpoint) by 84.3% (95% CI: 66.7, 92.6, p<0.001) (incidence rates: Enflonsia, 0.004; placebo, 0.024), showing increasing efficacy with increasing disease severity.
Approval was also supported by results from the Phase 3 SMART trial (MK-1654-007) evaluating the safety and efficacy of Enflonsia versus palivizumab in infants at increased risk for severe RSV disease. - Enflonsia is administered as a single 105 mg intramuscular (IM) injection for protection through a typical RSV season of 5 months.
- For infants born during the RSV season, Enflonsia is to be administered starting from birth.
- For infants born outside of the RSV season, Enflonsia should be administered prior to the start of their first RSV season.
The safety and effectiveness of Enflonsia have not been established in children older than 12 months of age. - Warnings and precautions associated with Enflonsia include serious hypersensitivity reactions, including anaphylaxis.
- Common adverse reactions include injection-site erythema (3.7%), injection-site swelling (2.7%) and rash (2.3%).
Development timeline for Enflonsia
Further information
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