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, represents a new option for preventing RSV illness in newborns and infants during or before their initial RSV season.
This approval comes at a time when the RSV treatment landscape has undergone significant evolution, particularly in therapies targeting older adults, who face potentially fatal outcomes from infections. While GSK, Pfizer, and Moderna have successfully introduced vaccines for the elderly population, addressing RSV in young children remains a critical need, as the virus continues to be the primary cause of infant hospitalizations across the United States.
Currently, vaccine options for young children are limited. The FDA’s decision last year to halt multiple RSV vaccine trials following severe cases in Moderna’s study has complicated the development landscape. Though Pfizer’s Abrysvo vaccine is available for expectant mothers to provide protection to their unborn children, direct vaccination of infants remains unavailable.
Clinical trials demonstrated Enflonsia’s effectiveness, showing a 60% reduction in RSV disease incidence and an 84% decrease in related hospitalizations over five months compared to placebo. The drug offers certain advantages over its main competitor, Sanofi and AstraZeneca’s Beyfortus, which has achieved blockbuster status since its 2023 approval. Notably, Enflonsia can be administered regardless of an infant’s weight and requires only a single injection, unlike Beyfortus’s two-shot regimen.
St. Jude Children’s Research Hospital’s Infectious Diseases Department Chair, Octavio Ramilo, who participated in the drug’s studies, highlighted Enflonsia’s combination of convenient dosing and robust clinical results as significant advantages for infant RSV protection.
The drug joins a limited arsenal of antibody treatments for infant RSV protection, which includes Synagis, a decades-old medication restricted to high-risk infants, and Beyfortus, which experienced such high demand upon release that Sanofi initially struggled with supply constraints. In response to potential market competition, Sanofi has announced plans to expedite Beyfortus shipments before the upcoming RSV season.
While Wall Street analysts have expressed uncertainty about
Enflonsia’s market potential, questioning whether its efficacy data significantly differentiates it from Beyfortus, Merck is positioning the drug as a competitive alternative in the infant RSV prevention space.
The Centers for Disease Control and Prevention’s advisory panel is scheduled to convene later this month to establish recommendations for Enflonsia’s use. Merck anticipates having supplies ready for distribution before the 2025-2026 RSV season begins.
Healthcare providers should note that Enflonsia carries
contraindications for infants with histories of serious
hypersensitivity reactions, including anaphylaxis, to any of its components. This safety consideration will be important in determining appropriate candidates for treatment.
The approval of Enflonsia marks another step forward in addressing the significant medical need for RSV prevention in infants, offering healthcare providers and parents an additional tool in protecting vulnerable newborns from this common but potentially serious respiratory infection. As the RSV treatment landscape continues to evolve, this new option could play a crucial role in reducing the burden of RSV-related hospitalizations among infants in the United States.
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