The Food and Drug Administration has approved a new drug from Merck designed to protect infants from respiratory syncytial virus, or RSV, a common but potentially serious seasonal illness.

The drug, Enflonsia, is a long-acting monoclonal antibody intended to prevent RSV-related lower respiratory tract disease in newborns and infants entering their first RSV season.

RSV is a leading cause of hospitalization among infants in the United States, especially during the fall and winter months, according to the Centers for Disease Control and Prevention.

Merck said Enflonsia is administered as a single 105 mg dose and provides protection for up to five months — the typical length of an RSV season.

“Enflonsia combines dosing convenience with strong clinical data showing significant reductions in RSV disease incidence and hospitalizations, making it a promising new intervention to help protect infants from RSV,” said Dr. Octavio Ramilo, chair of infectious diseases at St. Jude Children’s Research Hospital.

Merck said it plans to make the drug available in the U.S. ahead of the upcoming RSV season.

Enflonsia joins a growing arsenal of tools to prevent RSV in infants. Another option, the monoclonal antibody nirsevimab, was approved in 2023. It is recommended for all infants younger than 8 months who are entering their first RSV season if their mothers did not receive a maternal RSV vaccine.