The approval expands options for protecting infants from respiratory syncytial virus, which remains a leading cause of pediatric hospitalization.
Respiratory syncytial virus (RSV) disease is the leading cause of infant hospitalization in the US and can lead to serious respiratory conditions, like bronchiolitis and pneumonia.1 Preventive treatments for RSV in infants include long-acting monoclonal antibodies like nirsevimab.
On June 9, 2025, the FDA approved clesrovimab-cfor (Enflonsia), a long-acting monoclonal antibody, for the prevention of RSV lower respiratory tract disease (LRTD) in neonates and infants born during or entering their first RSV season.2 Developed by Merck, clesrovimab-cfor is designed to provide rapid, durable protection through a typical 5-month RSV season with a single intramuscular injection.2 The approval expands options for protecting infants from RSV, which remains a leading cause of pediatric hospitalization.
The FDA’s decision was supported by the CLEVER phase 2b/3 trial (MK-1654-004; NCT04767373), which enrolled 3614 preterm (gestational age, more than 29 to less than 35 weeks) and full-term infants (gestational age, more than 35 weeks). Infants were randomly assigned 2:1 to receive a single 105 mg intramuscular (IM) dose of clesrovimab-cfor or the placebo. Through 5 months of follow-up, clesrovimab-cfor reduced RSV associated with medically attended LRTD by 60.5% (95% CI, 44.2%-72.0%; P < .001). The antibody also reduced RSV-related hospitalizations by 84.3% (95% CI, 66.7%-92.6%, P < .001). A separate phase 3 trial, SMART (MK-1654-007; NCT04938830), compared clesrovimab-cfor to the older monthly antibody palivizumab in high‑risk infants. It demonstrated a safety profile comparable to palivizumab while allowing efficacy to be extrapolated from the CLEVER data.2
Clesrovimab-cfor is generally well tolerated. In clinical trials, the most common adverse reactions were injection‑site erythema (3.7%-3.8%), injection‑site swelling (2.7%), and rash (2.3%). These reactions were typically mild or moderate; more than 97% of adverse events were grade 1 or 2. Serious hypersensitivity reactions have been observed; therefore, clesrovimab-cfor is contraindicated in infants with a history of serious hypersensitivity, including anaphylaxis, to any of its components. Clinicians should monitor infants after administration and be prepared to treat allergic reactions. The FDA label notes that safety and efficacy have not been established in children older than 12 months.3
Clesrovimab-cfor is a single‑dose, 105 mg/0.7 mL prefilled syringe for IM injection. The same dose is administered to all infants regardless of weight. Infants born during the RSV season should receive clesrovimab-cfor soon after birth. Those born outside the RSV season should receive clesrovimab-cfor once prior to the start of their first RSV season.
Aryanna LeBron, PharmD, is a PGY-1 pharmacy resident at UConn John Dempsey Hospital in Farmington, Connecticut.
Kevin Chamberlin, PharmD, FASCP, is associate vice president and chief pharmacy officer at UConn Health in Farmington, Connecticut.
Infants undergoing cardiopulmonary bypass surgery require an additional 105 mg IM injection once they are stable after surgery to maintain adequate clesrovimab-cfor serum levels. The product must be refrigerated and allowed to reach room temperature before administration, and it should not be used if particulate matter or discoloration is present.2
Merck set the wholesale acquisition cost of clesrovimab-cfor at $556 per dose, matching the price of its main competitor, nirsevimab (Beyfortus). An economic analysis by the CDC’s Advisory Committee on Immunization Practices estimated an average cost of about $458 per dose, at $365 per dose for the Vaccines for Children Program and $560 per dose for other payers.4
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