Merck's Enflonsia Approval Sets 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 marks a significant development in the pharmaceutical industry's efforts to combat RSV, a leading cause of hospitalizations among infants in the United States.

Enflonsia vs. Beyfortus: A New Competitive Landscape

Merck's Enflonsia will directly compete with Sanofi and AstraZeneca's Beyfortus, which has dominated the infant RSV prevention market since its approval in July 2023. Beyfortus achieved blockbuster status in its first full commercial year, generating approximately $1.9 billion in sales.

Enflonsia's 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 and an 84.3% reduction in RSV hospitalizations compared to placebo in preterm and full-term infants up to 1 year old. These results position Enflonsia as a strong contender in the RSV prevention market.

While both Enflonsia and Beyfortus are monoclonal antibodies, they target different antigenic sites. Merck claims that Enflonsia may have an advantage in terms of dosing convenience, as it can be administered as a single 105 mg dose regardless of patient weight, whereas Beyfortus requires two different dosing options based on infant weight.

Market Dynamics and Supply Strategies

In response to Enflonsia's approval, Sanofi announced plans to accelerate shipping of Beyfortus 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 scheduled to start before the 2025-2026 RSV season. The company aims to capitalize on the growing demand for RSV prevention in infants, a market that has seen significant expansion since Beyfortus's introduction.

Regulatory and Advisory Developments

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. However, the recent unexpected move by Health Secretary Robert F. Kennedy Jr. to "retire" all 17 Committee members has created uncertainty about the meeting's composition and potential outcomes.

As the RSV prevention landscape continues to evolve, the pharmaceutical industry watches closely to see how Enflonsia's entry will reshape market dynamics and impact patient care in the coming RSV season.