The U.S. Food and Drug Administration has given its approval to a novel antibody treatment developed by Merck & Co. for protecting infants against respiratory syncytial virus (RSV). The medication, marketed as Enflonsia, received regulatory clearance on June 9, 2025, for use in newborns and infants entering their first RSV season.
The approval marks another significant advancement in RSV treatment options, which have expanded considerably in recent years. While several preventive therapies have emerged for older adults, including vaccines from GSK, Pfizer, and Moderna, treatment alternatives for infants have remained limited.
RSV continues to be the primary cause of infant hospitalizations in the United States. Currently, no RSV vaccines are available for young children, particularly after the FDA suspended multiple vaccine trials last year following reports of severe RSV cases in Moderna’s study. The only vaccine option in this space is Pfizer’s Abrysvo, which is administered to pregnant women to provide protection to their unborn children.
This limitation has made antibody treatments crucial for protecting young children. Before Enflonsia, two options existed: Synagis, a decades-old treatment restricted to high-risk infants, and Beyfortus, jointly developed by Sanofi and AstraZeneca, which achieved
blockbuster status after its 2023 approval.
Clinical trials demonstrated Enflonsia’s effectiveness, with the drug reducing RSV disease incidence by 60% compared to placebo and decreasing related hospitalizations by 84% over five months. While these results secured FDA approval, some Wall Street analysts question whether these outcomes significantly differ from Beyfortus’s performance, raising uncertainties about Enflonsia’s commercial prospects.
However, Merck’s new treatment offers distinct advantages in terms of accessibility. Unlike its competitor Beyfortus, Enflonsia can be administered regardless of an infant’s weight and requires only a single injection rather than two. These features could give the drug a competitive edge in the market.
Dr. Octavio Ramilo, who chairs the Department of Infectious Diseases at St. Jude Children’s Research Hospital and participated in the drug’s clinical studies, emphasized Enflonsia’s combination of convenient dosing and robust clinical results as key benefits for infant RSV protection.
The Centers for Disease Control and Prevention’s advisory panel is scheduled to meet later this month to establish usage recommendations for Enflonsia. Merck has confirmed that the medication will be available for distribution before the 2025-2026 RSV season begins.
The company has issued important safety information, noting that Enflonsia should not be administered to infants with a history of severe hypersensitivity reactions, including anaphylaxis, to any of the drug’s components.
This approval comes as Sanofi announced plans to expedite Beyfortus shipments to ensure widespread availability for the upcoming RSV season, indicating growing competition in the infant RSV prevention market.
The development represents a significant step forward in expanding treatment options for infant RSV protection, particularly given the limitations of current preventive measures for young children. With no vaccines available for direct administration to infants and young children, antibody treatments like Enflonsia provide an important tool for healthcare providers to protect this vulnerable population from severe RSV illness.
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