Merck's Enflonsia Approved by FDA, Setting Stage for RSV Market Battle

The U.S. Food and Drug Administration (FDA) has approved Merck's monoclonal antibody clesrovimab, to be marketed as Enflonsia, for the prevention of respiratory syncytial virus (RSV)-related lower respiratory tract disease in newborns and infants. This approval positions Enflonsia as a direct competitor to Sanofi and AstraZeneca's blockbuster RSV antibody, Beyfortus, in a rapidly evolving market for infant RSV prevention.

Enflonsia's Approval and Clinical Data

Enflonsia's FDA approval is based on data from the Phase IIb/III CLEVER study, which demonstrated a 60.5% reduction in RSV-associated medically attended lower respiratory infections compared to placebo in preterm and full-term infants up to 1 year of age. The study also showed an 84.3% reduction in RSV hospitalizations versus placebo.

Additional support came from the Phase III SMART trial, which compared Enflonsia to Sobi's Synagis, another monoclonal antibody for RSV prevention. Interim results from October 2024 indicated that Enflonsia could match Synagis's safety and efficacy profiles through 5 months of observation.

Merck's Enflonsia is administered as a single 105 mg dose, regardless of patient weight, and provides protection for five months, typically covering the duration of an RSV season.

Market Competition and Availability

Enflonsia will directly compete with Sanofi and AstraZeneca's Beyfortus, which was approved in July 2023 and has since become a top-performing asset. Beyfortus achieved blockbuster status in its first full commercial year with €1.7 billion (approximately $1.9 billion) in sales.

In response to Enflonsia's approval, Sanofi announced plans to accelerate shipping of Beyfortus, aiming to ensure broad availability ahead of the 2025-2026 RSV season. Shipments are set to begin in early Q3 to aid advance immunizations.

Merck expects orders for Enflonsia to begin in July, with shipments starting before the 2025-2026 RSV season. The Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices is scheduled to meet later this month to discuss recommendations for Enflonsia's use.

Broader RSV Prevention Landscape

The approval of Enflonsia adds to a growing arsenal of RSV prevention tools. For older adults, three vaccines are now available from GSK (Arexvy), Pfizer (Abrysvo), and Moderna (mRESVIA). However, there are currently no RSV vaccines available for young children, with the FDA having paused multiple trials of RSV shots following cases of severe RSV in a Moderna study.

Pfizer's Abrysvo is available for pregnant women to pass RSV protection to newborns, offering a different approach to infant protection. Its potential advantage lies in its lower production cost as a vaccine compared to monoclonal antibodies.

As pharmaceutical companies prepare for the upcoming RSV season, the market for infant RSV prevention is poised for increased competition, with potential benefits for accessibility and patient care.