FDA Approves Merck’s Enflonsia for RSV Prevention in Infants

The U.S. Food and Drug Administration has approved Merck’s monoclonal antibody, clesrovimab, which will be marketed as Enflonsia, for the prevention of lower respiratory tract disease caused by respiratory syncytial virus (RSV) in infants entering or born during their first RSV season. The authorization came one day ahead of the agency’s scheduled decision deadline.
Enflonsia will be introduced as a competitor to Beyfortus, the RSV antibody developed by Sanofi and AstraZeneca, which has held a leading position in infant RSV prophylaxis. Enflonsia is designed for use in healthy full-term, pre-term, and at-risk infants, regardless of birth weight, and is administered as a single dose. The treatment is priced at $556 per dose.

RSV infections in the United States typically rise from November through April. Though symptoms often resemble a severe cold, RSV can lead to serious outcomes such as pneumonia, particularly in infants, the elderly, and immunocompromised individuals. In the U.S., approximately 58,000 to 80,000 children under five are hospitalized with RSV annually, with an estimated 100 to 500 fatalities in that age group. Globally, the virus causes hospitalization for around 3 million children under age five and approximately 100,000 deaths each year.
Merck’s late-stage trial results for Enflonsia showed an 84% reduction in RSV-related hospitalizations through five months when compared to a placebo. The same study met its primary goal by showing a 60% decrease in medically attended lower respiratory infections (MALRI) requiring signs of lower respiratory involvement. The trial included both preterm and full-term infants up to one year of age.
Enflonsia’s clinical profile is supported by data indicating a comparable safety profile to Synagis, another RSV antibody. It also offers an advantage over Beyfortus in terms of dosing simplicity, as the latter requires two weight-based dosing options, making forecasting and inventory planning more complex.
Sanofi and AstraZeneca reported that demand for Beyfortus exceeded initial supply in 2023. In response, the companies have since increased production, now claiming to have already manufactured a volume of doses equal to last year’s supply and have announced plans to begin shipping in the third quarter. The total supply is expected to surpass previous levels.
In contrast, Merck stated it anticipates delivery of Enflonsia ahead of the 2025–2026 RSV season. The CDC’s Advisory Committee on Immunization Practices is scheduled to review Enflonsia, alongside other immunization products, during a meeting from June 25 to 27. However, the recent dismissal of all CDC panel members by Health Secretary Robert F. Kennedy Jr. has created uncertainty regarding the meeting.
Beyfortus showed a 75% reduction in MALRI in clinical data, and its European label includes protection duration of up to six months. While both Enflonsia and Beyfortus are monoclonal antibodies, they target different antigenic sites, which limits direct comparisons of their efficacy. Additionally, Merck has suggested that RSV may develop resistance more readily to the antigenic site targeted by Beyfortus.
Other participants in the RSV market include Pfizer’s Abrysvo, which is administered to pregnant individuals during weeks 32 to 36 of gestation to confer passive immunity to newborns. As a vaccine, Abrysvo’s production cost may be lower compared to monoclonal antibodies, making it a competitive alternative.