Merck wins FDA approval for Enflonsia, sparking RSV market competition with Sanofi and AstraZeneca's Beyfortus

The U.S. Food and Drug Administration (FDA) has granted approval to Merck's new monoclonal antibody, known commercially as Enflonsia, to prevent lower respiratory tract disease caused by respiratory syncytial virus (RSV) in infants. This important decision places Merck in direct competition with Sanofi and AstraZeneca's existing RSV treatment, Beyfortus, as several major pharmaceutical companies prepare for the upcoming RSV season.

RSV, which often presents symptoms similar to a severe cold, can become life-threatening for infants, the elderly, and individuals with compromised immune systems. In the United States, RSV infections typically emerge in November and persist through April, making timely prevention crucial. The Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices is scheduled to convene in late June to establish usage guidelines for Enflonsia and other viral treatments. Merck anticipates being ready to distribute Enflonsia before the next RSV season commences.

The competition in the RSV treatment space is intensifying, with GSK’s Arexvy and Pfizer’s Abrysvo leading the market for RSV vaccines for older adults. Moderna's recently approved mRESVIA is expected to join them in the coming RSV season. Until now, Sanofi and AstraZeneca have faced no competition in the pediatric RSV antibody market, with Beyfortus being the sole option for children. Last year, Beyfortus achieved substantial sales of €1.7 billion ($1.8 billion), although high demand reportedly exceeded initial supply. Sanofi and AstraZeneca have announced plans to match last year's supply levels and are preparing to ship their product early in the third quarter.

The clinical trials for Enflonsia have demonstrated promising results. Eight months ago, Merck disclosed that clesrovimab, the scientific name for Enflonsia, reduced RSV-associated hospitalizations by 84% compared to a placebo over a five-month period, successfully reaching a secondary endpoint in a late-stage trial. The study also met its primary endpoint, showing a 60% decrease in RSV-associated medically attended lower respiratory infections (MALRI) compared to a placebo. The trial included both preterm and full-term infants up to one year old. Although direct comparisons between Enflonsia and Beyfortus are challenging due to differences in their respective trials, both drugs are monoclonal antibodies that target distinct antigenic sites.

One potential advantage of Enflonsia is its single-dose regimen, compared to Beyfortus, which requires two different dosing options based on the infant's weight. This convenience might be a significant factor in Merck's favor, along with its claims that the virus may develop resistance more readily to Sanofi and AstraZeneca's treatment. Dr. Octavio Ramilo from St. Jude Children’s Research Hospital highlighted Enflonsia’s efficacy and convenience, describing it as a promising new option for infant RSV protection.

Sanofi, however, has emphasized Beyfortus’ performance, citing a 75% reduction in MALRI incidence compared to Enflonsia’s 60%. Moreover, Sanofi points to the extended six-month protection duration of Beyfortus, now included on its new European label.

In addition to the monoclonal antibodies, Pfizer's Abrysvo offers a unique approach by allowing administration to pregnant women between the 32nd and 36th weeks of pregnancy to protect their newborns. Abrysvo’s cost-effectiveness as a vaccine could lead to broader adoption, as it is less expensive to manufacture than monoclonal antibodies.

In the U.S., RSV is responsible for the hospitalization of about 58,000 children under age 5 each year, with 100 to 500 resulting deaths. Globally, approximately 3 million children in this age group are hospitalized annually due to RSV, and 100,000 die from the infection, making it the second leading cause of death after malaria in low- and middle-income countries.