Merck's Enflonsia Approved for RSV, Setting Stage for Market Battle with Sanofi and AstraZeneca's Beyfortus

The U.S. Food and Drug Administration (FDA) has approved Merck's monoclonal antibody drug Enflonsia (clesrovimab) for the prevention of respiratory syncytial virus (RSV) in infants, marking a significant development in the treatment landscape for this widespread pathogen. This approval positions Enflonsia as a direct competitor to Sanofi and AstraZeneca's blockbuster drug Beyfortus, which has dominated the infant RSV market since its clearance in 2023.

FDA Approval and Clinical Efficacy

Enflonsia received FDA approval for use in newborns and infants born during or entering their first RSV season. In clinical trials, the drug demonstrated impressive efficacy, reducing the incidence of RSV-associated medically attended lower respiratory infections by 60% compared to placebo. More notably, Enflonsia showed an 84% reduction in RSV-related hospitalizations over a five-month period.

Dr. Octavio Ramilo, chair of the Department of Infectious Diseases at St. Jude Children's Research Hospital and a study investigator, highlighted the drug's potential impact: "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."

Market Implications and Competition

The approval of Enflonsia sets the stage for an intense market battle in the infant RSV prevention space. Sanofi and AstraZeneca's Beyfortus, approved in 2023, quickly became a blockbuster medicine, generating sales of 1.7 billion euros ($1.8 billion) last year. The demand for Beyfortus was so strong that it initially outpaced supply, prompting Sanofi to boost its manufacturing capabilities.

In response to Merck's entry into the market, Sanofi has announced plans to accelerate shipping of Beyfortus to ensure broad availability ahead of the coming RSV season. The company stated that it has already manufactured as many doses as were supplied last year, with production ongoing and shipments set to begin early in the third quarter.

While both Enflonsia and Beyfortus are monoclonal antibodies, they target different antigenic sites, making direct comparisons challenging. However, Merck's drug may offer some advantages, including a single-dose administration regardless of an infant's weight, compared to Beyfortus's two different dosing options based on weight.

Broader RSV Treatment Landscape

The approval of Enflonsia comes amid significant changes in the RSV treatment landscape, particularly for older adults. Three vaccines are now available from GSK, Pfizer, and Moderna for this demographic. However, for young children, options remain limited. There are currently no RSV vaccines available for this age group, with the FDA having paused multiple trials of RSV shots following cases of severe RSV in a Moderna study.

Pfizer's Abrysvo, a vaccine for pregnant women that can pass RSV protection to newborns, represents another approach to infant protection. Its potential for more widespread use lies in its lower production costs compared to monoclonal antibodies.

As the 2025-2026 RSV season approaches, the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices is scheduled to meet later this month to discuss recommendations for using Enflonsia. Merck expects to have the drug available for shipment before the start of the season, potentially reshaping the market dynamics in infant RSV prevention.