With the U.S. FDA’s approval of Enflonsia (clesrovimab) 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. The preventive, long-acting monoclonal antibody (MAb) will take its place in the market alongside the blockbuster Beyfortus (nirsevimab) from Sanofi SA and Astrazeneca plc. The MAb for pediatric use brought in about $1.77 billion in 2024.