FDA

FDA greenlights second monoclonal antibody for RSV prevention in infants

June 12, 2025

Brand name: Enflonsia

Generic name: clesrovimab-cfor

Manufacturer: Merck

Approval date: June 9, 2025

FDA approved Enflonsia (clesrovimab-cfor), an RSV F protein-directed fusion inhibitor, for the prevention of RSV lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.

Efficacy

Approval was based on data from the phase 2b/3 CLEVER trial (NCT04767373) that evaluated a single dose of Enflonsia administered to preterm and full-term infants (birth to 1 year of age). The trial met its primary and key secondary endpoints:

• Enflonsia led to a reduction in incidence of RSV-associated medically attended lower respiratory infections (MALRI) requiring ≥1 indicator of lower respiratory infection (LRI) or severity compared with placebo through 5 months (primary endpoint) by 60.5% (95% confidence interval [CI], 44.2-72.0; p<0.001) (incidence rates: Enflonsia, 0.026; placebo, 0.065).
• Enflonsia also showed 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 (NCT04938830) evaluating the safety and efficacy of Enflonsia vs. palivizumab in infants at increased risk for severe RSV disease.

The incidence rate of RSV-associated MALRI (requiring ≥1 indicator of LRI or severity) through 150 days after dosing was generally comparable between Enflonsia (incidence rate, 3.6%; 95% CI, 2.0-6.0) and palivizumab (incidence rate, 2.9%; 95% CI, 1.5-5.2). The incidence rate of RSV-associated hospitalization through 150 days after dosing was also generally comparable between Enflonsia (incidence rate, 1.3%; 95% CI, 0.4-2.9) and palivizumab (incidence rate, 1.5%; 95% CI, 0.5-3.2).

Safety

The most common adverse reactions were injection-site erythema (3.7%), injection-site swelling (2.7%), and rash (2.3%).

Recommended dose

The recommended dosage of Enflonsia is 105 mg via a single IM injection, regardless of the infant's weight.

Sources:

Enflonsia (clesrofimab-cfor) [package insert]. Food and Drug Administration. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/761432s000lbl.pdf Revised June 2025. Accessed June 11, 2025.

US FDA approves Merck’s Enflonsia™ (clesrovimab-cfor) for prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in infants born during or entering their first RSV season. [News release]. 2025. https://www.merck.com/news/u-s-fda-approves-mercks-enflonsia-clesrovimab-cfor-for-prevention-of-respiratory-syncytial-virus-rsv-lower-respiratory-tract-disease-in-infants-born-during-or-entering-their-fir/