Merck's Enflonsia Gains FDA Approval, Challenging Sanofi and AstraZeneca in RSV Prevention Market

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) lower respiratory tract disease in newborns and infants. This approval sets the stage for a competitive battle in the RSV prevention market, particularly against Sanofi and AstraZeneca's blockbuster drug Beyfortus.

Enflonsia's Approval and Clinical Data

Enflonsia is indicated for use in newborns and infants born during or entering their first RSV season. The approval was based on data from the Phase IIb/III CLEVER study, which demonstrated that Enflonsia reduced RSV-associated medically attended lower respiratory infections by 60.5% compared to placebo in preterm and full-term infants up to 1 year of age. Additionally, the drug cut RSV hospitalizations by 84.3% versus placebo.

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

Market Competition and Distribution Plans

Merck's entry into the RSV prevention market will directly challenge 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 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 ahead of the 2025-2026 season. The company emphasizes Enflonsia's dosing convenience, as it can be administered as a single 105 mg dose regardless of patient weight, potentially offering an advantage over Beyfortus' weight-based dosing regimen.

Regulatory and Advisory Committee 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, pharmaceutical companies are positioning themselves to capture market share in this critical area of infant health. The introduction of Enflonsia marks a significant development in the ongoing efforts to protect vulnerable populations from the potentially severe consequences of RSV infections.