Merck's Enflonsia Enters RSV Prevention Market, Challenging Sanofi and AstraZeneca's Beyfortus

The Food and Drug Administration (FDA) has approved Merck's new antibody drug, Enflonsia (clesrovimab), for the prevention of respiratory syncytial virus (RSV) in newborns and infants. This approval marks a significant development in the RSV treatment landscape, setting the stage for increased competition in a market currently dominated by Sanofi and AstraZeneca's Beyfortus.

FDA Approval and Drug Details

Enflonsia, formerly known as clesrovimab, received FDA clearance on June 9, 2025, one day ahead of the agency's target decision date. The monoclonal antibody is approved to prevent lower respiratory tract disease caused by RSV in children born during or entering their first RSV season.

Merck's new drug offers a single-dose administration, regardless of the infant's weight, which may provide an advantage over Beyfortus, which requires two different dosing options depending on the baby's weight. The company plans to begin shipments before the start of the 2025-2026 RSV season, pending recommendations from the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices (ACIP).

Clinical Efficacy and Market Implications

In clinical trials, Enflonsia demonstrated significant efficacy in reducing RSV-related hospitalizations and disease incidence. The drug reduced the incidence of RSV-associated medically attended lower respiratory infections (MALRI) by 60% compared to placebo over five months. More importantly, it decreased RSV-related hospitalizations by 84% during the same period.

These results position Enflonsia as a strong competitor to Beyfortus, which has quickly become a blockbuster medicine since its approval in 2023. However, direct comparisons between the two drugs are complicated by differences in their respective clinical trials and the fact that they target different antigenic sites of the RSV virus.

Sanofi and AstraZeneca have highlighted Beyfortus' 75% reduction in MALRI incidence and its six-month protection duration, now reflected on its European label. The companies have also announced plans to accelerate shipping of Beyfortus to ensure broad availability for the upcoming RSV season, having already manufactured as many doses as they supplied last year.

Expanding RSV Prevention Landscape

The approval of Enflonsia adds to a rapidly evolving RSV prevention market. While several vaccines are now available for older adults, including GSK's Arexvy, Pfizer's Abrysvo, and Moderna's recently approved mRESVIA, options for infants remain limited.

Pfizer's Abrysvo, approved for use in pregnant women, offers another approach to infant protection. Its potential for more widespread use lies in its lower production costs as a vaccine compared to monoclonal antibodies.

As pharmaceutical companies continue to innovate in this space, the ultimate beneficiaries are the millions of infants at risk of severe RSV infections. In the United States alone, approximately 58,000 children under age 5 are hospitalized annually due to RSV, with 100 to 500 deaths occurring each year in this age group. Globally, the impact is even more significant, with an estimated 3 million hospitalizations and 100,000 deaths among children under 5 years old.