With the U.S. FDA’s approval of Enflonsia (cesrovimab) to prevent respiratory syncytial virus (RSV) infection lower respiratory tract disease in newborns and infants, Merck & Co. Inc. steps into a space of competition and regulatory shifts.

Merck & Co. said it won US approval for a shot that protects against RSV, the most common cause of hospitalization among infants.

Known as Enflonsia, the drug will compete for market share with Sanofi and AstraZeneca’s in-demand Beyfortus, which quickly became a blockbuster medicine following its approval two years ago.