Merck’s Enflonsia will go up against Sanofi and AstraZeneca’s Beyfortus, which the partners plan to ship out early in the third quarter.
The FDA approved the use of Merck’s anti-respiratory syncytial virus antibody clesrovimab in infants on Monday. Merck will market the therapy under the brand name Enflonsia.
The approval covers the use of Enflonsia to prevent RSV-related lower respiratory tract disease in newborns and infants who are entering their first season of circulating respiratory diseases. According to Merck, Enflonsia provides “rapid and durable” RSV protection persisting for five months, the length of a typical RSV season. Enflonsia is dosed at 105 mg regardless of patient weight.
With Enflonsia, Merck will now go up against Sanofi and AstraZeneca, which own the immunizing antibody Beyfortus. Approved in July 2023, Beyfortus has since become a top-performing asset for the companies, hitting blockbuster status in its first full commercial year with €1.7 billion (approximately $1.9 billion) in sales.
In March 2024, the CDC released data from almost 700 infants showing that Beyfortus was 90% effective at preventing hospitalization associated with RSV infections. The agency currently recommends the use of Beyfortus in infants 8 months and younger.
On Monday, Sanofi announced it would ship Beyfortus out early “to ensure broad availability” ahead of the 2025-2026 RSV season. Shipments will begin in “early Q3” to aid advance immunizations, the pharma noted.
Enflonsia’s approval on Monday was backed by data from the Phase IIb/III CLEVER study, which found that in preterm and full-term infants up to 1 year, the antibody reduced RSV-associated medically attended lower respiratory infections by 60.5% versus placebo. Enflonsia also cut RSV hospitalizations by 84.3% as compared with placebo.
Additional supporting data came from the Phase III SMART trial, which compared Enflonsia against Sobi’s Synagis, which is also a monoclonal antibody designed to prevent RSV. Interim results announced in October 2024 showed that Enflonsia could match Synagis’ safety and efficacy profiles through 5 months of observation.
According to Merck’s Monday announcement, the CDC’s Advisory Committee on Immunization Practices is scheduled to meet “later this month” to discuss the use of Enflonsia in infants. It is yet unclear how Health Secretary Robert F. Kennedy Jr.’s sudden move of “retiring” all 17 Committee members on Monday evening will affect this meeting. In a Tuesday morning note to investors, William Blair wrote that the meeting will “proceed as scheduled though no guidance was given regarding the expected composition of the ACIP for that meeting.”
Merck expects orders for Enflonsia to begin in July, with shipments set to start ahead of the 2025-2026 season.
