Clesrovimab, a monoclonal antibody for the prevention of respiratory syncytial virus disease in infants: a drug profile.

INTRODUCTION: Respiratory syncytial virus (RSV) is a leading cause of acute lower respiratory tract infection (LRTI) in young children. In children < 5 years old, it causes an estimated 33 million LRTI episodes and over 100,000 deaths annually, two-thirds of which occur in infants. The licensure of new long-acting monoclonal antibodies and a maternal RSV vaccine offer opportunities for reducing RSV related infant morbidity and mortality. AREAS COVERED: This drug profile reviews clesrovimab (Enflonsia, Merck & Co. Inc. Rahway, NJ U.S.A.), an extended half-life human IgG1κ monoclonal antibody, targeting the highly conserved antigenic site IV of the RSV fusion protein. We synthesize evidence from phase 1 to phase 3 clinical trials, pharmacokinetic and pharmacodynamic data, regulatory approvals and resistance profiling, drawing on peer-reviewed literature and regulatory sources available until May 2026. EXPERT OPINION: Clesrovimab offers a single, fixed-dose, weight-independent injection per RSV season, with demonstrated safety and efficacy against medically attended RSV disease and hospitalization in infants. Its epitope target is distinct from that of nirsevimab (site Ø) and no cross resistance between the two antibodies has been observed in vitro. Equitable global access to monoclonal antibodies, particularly for low- and middle-income countries, remains critical to reduce the global RSV burden.