A monoclonal antibody vaccine that’s protective against respiratory syncytial virus (RSV), which can cause severe illness in children under 5, has come to market — but supplies are more limited than demand, so far.

For now, the Centers for Disease Control and Prevention is advising doctors to prioritize immunization of babies at the highest risk of severe disease: Infants under 6 months who have chronic lung disease or other underlying conditions. RSV is the second leading cause of hospitalization in the nation, sending about 80,000 keiki ages 5 and younger to hospitals each year.