MSD Reports Positive Phase III RSV Drug Data
What happened
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.
Why it matters
- 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%.
Key numbers
- 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.
What happens next
- 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.
Quick answers
What happened in 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.
Why does MSD Reports Positive Phase III RSV Drug Data matter?
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%.
