MSD Reports Positive Phase III RSV Drug Data
MSD (Merck) has released positive results from its Phase III SMART trial for Enflonsia, a prophylaxis for Respiratory Syncytial Virus (RSV) in infants. The data, collected across two RSV seasons, showed a consistent safety profile for children under two. The intervention also demonstrated a significant risk reduction for RSV-related hospitalizations and severe illness.
- Enflonsia, with the active ingredient clesrovimab-cfor, is a long-acting monoclonal antibody, not a vaccine. It provides passive immunity by targeting the RSV F protein, which prevents the virus from entering cells and replicating. - The U.S. Food and Drug Administration (FDA) approved Enflonsia on June 9, 2025. This approval was based on data from the pivotal Phase 2b/3 CLEVER trial and interim results from the first season of the Phase III SMART trial. - Enflonsia will compete with Beyfortus (nirsevimab), another single-dose monoclonal antibody for infant RSV prevention developed by Sanofi and AstraZeneca that was approved in July 2023. - RSV is the leading cause of hospitalization for infants in the United States, with an estimated 80,000 hospitalizations occurring annually in children under one year old. - The SMART trial compared Enflonsia to palivizumab, an older monoclonal antibody which was the previous standard of care for high-risk infants but required monthly injections throughout the RSV season. - Enflonsia is administered as a single intramuscular injection designed to provide protection through a typical 5-month RSV season. - The broader RSV prevention strategy for infants now includes multiple options: either a maternal RSV vaccine (Pfizer's Abrysvo) administered during pregnancy or an infant-administered monoclonal antibody like Enflonsia or nirsevimab after birth. - Real-world data from the 2024-2025 season, the first with widespread availability of a maternal vaccine and nirsevimab, showed significant reductions in RSV-associated infant hospitalizations, with rates for infants under 8 months dropping by 28% to 43%.
