The U.S. Food and Drug Administration has approved Merck’s monoclonal antibody therapy, clesrovimab (brand name Enflonsia), for preventing respiratory syncytial virus (RSV) in infants up to one year old during their first RSV season. Enflonsia is the first and only single-dose preventive shot approved for use regardless of birth weight in healthy, pre-term, full-term, and high-risk infants. Merck announced the treatment will be priced at $556 per dose.
RSV is a common seasonal virus that can lead to serious respiratory illness, including pneumonia. It is one of the leading causes of hospitalization and death in infants and older adults. The U.S. Centers for Disease Control and Prevention (CDC) estimates that RSV hospitalizes 58,000 to 80,000 children under five each year.
Enflonsia’s approval follows positive results from a late-stage trial that demonstrated a safety profile comparable to Synagis, a monthly RSV antibody shot by Swedish Orphan Biovitrum. Unlike Synagis and Sanofi–AstraZeneca’s Beyfortus, which requires weight-based dosing, Enflonsia offers a simplified, single-dose regimen that could ease clinical logistics and inventory management for healthcare providers.
The CDC currently recommends two options for infant RSV protection: vaccination during pregnancy or administration of an RSV antibody post-birth. Merck expects Enflonsia to be available in time for the 2025–2026 RSV season.
Merck also noted that the CDC’s Advisory Committee on Immunization Practices is expected to review usage recommendations for Enflonsia later this month. However, recent reports indicate that U.S. Health Secretary Robert F. Kennedy Jr. has dismissed the current CDC panel members.