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 comes at a time when RSV prevention has seen significant advances, particularly for older adults who now have access to three different vaccines manufactured by GSK, Pfizer, and Moderna. However, young children remain especially vulnerable to RSV, which stands as the primary cause of infant hospitalizations in the United States.
Currently, vaccination options for young children are limited. The FDA previously halted multiple RSV vaccine trials following reports of severe cases in Moderna’s study. While Pfizer’s Abrysvo vaccine is available for expectant mothers to provide protection to their unborn children, direct vaccination of infants is not yet possible.
Parents currently rely on antibody treatments for protecting their young children against RSV. Until recently, options were limited to Synagis, a decades-old treatment restricted to high-risk infants, and Beyfortus, jointly developed by Sanofi and AstraZeneca. Beyfortus, which received approval in 2023, quickly achieved blockbuster status, though initial supply constraints forced Sanofi to enhance its production capacity.
Clinical trials demonstrated Enflonsia’s effectiveness, with the drug reducing RSV disease incidence by 60% compared to placebo over five months, while decreasing RSV-related hospitalizations by 84%. Despite these positive results, some Wall Street analysts question whether these outcomes significantly differentiate the drug from Beyfortus’s performance, raising uncertainties about its commercial prospects.
However, Enflonsia offers certain advantages over its competitor. The drug can be administered regardless of an infant’s weight and requires only a single injection, compared to Beyfortus’s two-dose regimen. Sanofi has responded to the new competition by announcing accelerated Beyfortus shipments to ensure widespread availability before the upcoming RSV season.
St. Jude Children’s Research Hospital’s Infectious Diseases Department Chair, Dr. Octavio Ramilo, who participated in the drug’s clinical studies, praised Enflonsia’s combination of convenient dosing and robust clinical results in preventing RSV disease and
hospitalizations.
The Centers for Disease Control and Prevention’s advisory committee will convene later this month to discuss recommended guidelines for Enflonsia’s use. Merck anticipates making the treatment available before the 2025-2026 RSV season begins.
The company has issued important safety information, warning 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 development represents a significant addition to the limited arsenal of RSV preventive treatments for infants. With RSV continuing to pose a serious health threat to young children, the availability of another preventive option could help reduce the burden of this respiratory illness on families and healthcare systems.
The approval of Enflonsia also marks an important milestone in the evolving landscape of RSV prevention, particularly for the vulnerable infant population. As the medical community awaits the CDC’s recommendations, this new treatment option could potentially reshape how healthcare providers approach RSV prevention in young children, offering an alternative to existing therapies while providing parents with more choices for protecting their infants against this common but potentially serious respiratory virus.
Be First to Comment