FDA Approves Merck’s RSV Shot for Infants, Challenging Sanofi and AstraZeneca’s Market Hold

The U.S. Food and Drug Administration has approved Merck’s respiratory syncytial virus (RSV) shot for infants, adding a new competitor to a critical space dominated by Sanofi and AstraZeneca.

The newly approved drug, branded as Enflonsia, is intended to protect infants during their first RSV season, which typically begins in the fall. Merck said shipments are expected to begin in July to ensure early availability before the virus circulates widely.

Enflonsia will compete directly with Beyfortus, the leading monoclonal antibody treatment developed by Sanofi and AstraZeneca. The latter faced nationwide shortages last year due to overwhelming demand.

“Merck is committed to easing the burden of RSV on families and healthcare systems,” said Dr. Dean Li, president of Merck Research Laboratories.

Both Enflonsia and Beyfortus are preventative monoclonal antibodies but differ in how they target the virus. Merck’s treatment offers a fixed-dose approach regardless of an infant’s weight—a potential advantage over Beyfortus, which requires weight-based dosing.

RSV is one of the leading causes of hospitalization in infants and can also be deadly among older adults. In clinical trials, Enflonsia demonstrated an 84% reduction in RSV-related hospitalizations and a 90% reduction in hospitalizations due to lower respiratory tract infections.

With Pfizer, GSK, and Moderna offering adult or maternal RSV vaccines, Merck’s entry with an infant-focused solution intensifies the competition. All manufacturers await CDC guidance later this month on immunization strategies for the upcoming season.