New FDA-approved monoclonal antibody from Merck offers simplified protection against RSV for infants with a single dose regardless of weight, potentially transforming preventive care for America’s youngest citizens.
Key Takeaways
- Merck’s Enflonsia (clesrovimab) has received FDA approval to protect infants against RSV, with a single 105 mg dose providing up to five months of protection
- Unlike competitor Beyfortus, Enflonsia uses one standard dose for all babies regardless of weight, simplifying administration for healthcare providers
- Clinical trials demonstrated Enflonsia reduces medically attended lower respiratory infections by over 60% and RSV hospitalizations by over 84%
- Priced at $556 per dose, Enflonsia will be available for the 2025 RSV season with orders beginning in July
Simplified Protection Against a Leading Threat
The FDA has approved Merck’s monoclonal antibody Enflonsia (clesrovimab) for the prevention of respiratory syncytial virus (RSV) in infants born during or entering their first RSV season. This approval, announced on June 9, marks a significant advancement in protecting America’s youngest and most vulnerable citizens from what stands as the leading cause of hospitalization among U.S. infants. Unlike existing options, Enflonsia offers a straightforward approach with a single 105 mg dose that provides protection for up to five months, regardless of the infant’s weight.
“ENFLONSIA provides an important new preventive option to help protect healthy and at-risk infants born during or entering their first RSV season with the same dose regardless of weight,” said Dr. Dean Y. Li, president of Merck Research Laboratories.
The simplicity of Merck's monoclonal antibody for RSV in infants could be a plus in its competition with AstraZeneca's existing treatment. https://t.co/Lhb58aI0Od
— STAT (@statnews) June 9, 2025
Competitive Advantages in the RSV Prevention Market
Enflonsia enters a relatively new market for RSV prevention in infants, positioning itself as a simpler alternative to Sanofi and AstraZeneca’s Beyfortus (nirsevimab), which requires different dosages based on infant weight. Priced competitively at $556 per dose, Enflonsia matches Beyfortus’s cost while potentially offering significant logistical advantages for healthcare providers. This single-dose approach eliminates the need for weight-based calculations and reduces the potential for dosing errors, streamlining the administrative process in busy clinical settings.
“We do think that the fact … that we have a single dose for all babies regardless of weight will just help some of those logistical issues that nirsevimab was faced with during its rollout,” said Paula Annunziato, Merck’s head of vaccines clinical research.
The introduction of Enflonsia comes at a time when HHS Secretary Robert F. Kennedy Jr. has replaced all 17 voting members of the CDC’s Advisory Committee on Immunization Practices (ACIP), which will make recommendations for the drug’s use in late June. This reshaping of the committee under President Trump’s administration signals a potentially more streamlined approach to preventive medicine approvals compared to previous administrations.
Impressive Clinical Efficacy and Safety Profile
The FDA’s approval of Enflonsia is backed by robust clinical evidence from the Phase 2b/3 CLEVER trial and the Phase 3 SMART trial. These studies demonstrated that the monoclonal antibody reduced medically attended lower respiratory tract infections by over 60% and RSV hospitalizations by over 84% compared to existing methods. These impressive results underscore the potential public health impact of widespread Enflonsia adoption, especially considering that RSV is the leading cause of hospitalization among U.S. infants.
“RSV disease is the leading cause of infant hospitalization in the U.S. and can lead to serious respiratory conditions like bronchiolitis and pneumonia,” explained Dr. Octavio Ramilo, Chief of the Division of Infectious Diseases at Nationwide Children’s Hospital.
Market Availability and Future Expansion
Merck plans to begin taking orders for Enflonsia in July, with shipments expected to arrive in time for the 2025 RSV season. Currently, Enflonsia is approved only for use in infants during their first RSV season, while Beyfortus is approved for high-risk babies in their second RSV season as well. However, Merck is conducting a study comparing Enflonsia to Synagis, another RSV monoclonal antibody, to potentially extend its use to high-risk children in their second RSV season, further expanding its market potential.
“ENFLONSIA combines dosing convenience with strong clinical data showing significant reductions in RSV disease incidence and hospitalizations, making it a promising new intervention to help protect infants from RSV,” added Dr. Octavio Ramilo.
The availability of multiple RSV preventive options, including Enflonsia and Pfizer’s Abrysvo vaccine for pregnant women, represents a significant advancement in protecting vulnerable infants. An ACIP work group has highlighted the benefits of having multiple products to prevent resistance and address supply and pricing issues, ensuring American families have access to the best possible protection for their children during RSV season.
