Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infection in infants, and a need exists for prevention of RSV in healthy infants. Nirsevimab is a monoclonal antibody with an extended half-life that is being developed to protect infants for an entire RSV season with a single intramuscular dose.In this trial conducted in both northern and southern hemispheres, we evaluated nirsevimab for the prevention of RSV-associated lower respiratory tract infection in healthy infants who had been born preterm (29 weeks 0 days to 34 weeks 6 days of gestation). We randomly assigned the infants in a 2:1 ratio to receive nirsevimab, at a dose of 50 mg in a single intramuscular injection, or placebo at the start of an RSV season. The primary end point was medically attended RSV-associated lower respiratory tract infection through 150 days after administration of the dose. The secondary efficacy end point was hospitalization for RSV-associated lower respiratory tract infection through 150 days after administration of the dose.From November 2016 through November 2017, a total of 1453 infants were randomly assigned to receive nirsevimab (969 infants) or placebo (484 infants) at the start of the RSV season. The incidence of medically attended RSV-associated lower respiratory tract infection was 70.1% lower (95% confidence interval [CI], 52.3 to 81.2) with nirsevimab prophylaxis than with placebo (2.6% [25 infants] vs. 9.5% [46 infants]; P<0.001) and the incidence of hospitalization for RSV-associated lower respiratory tract infection was 78.4% lower (95% CI, 51.9 to 90.3) with nirsevimab than with placebo (0.8% [8 infants] vs. 4.1% [20 infants]; P<0.001). These differences were consistent throughout the 150-day period after the dose was administered and across geographic locations and RSV subtypes. Adverse events were similar in the two trial groups, with no notable hypersensitivity reactions.A single injection of nirsevimab resulted in fewer medically attended RSV-associated lower respiratory tract infections and hospitalizations than placebo throughout the RSV season in healthy preterm infants. (Funded by AstraZeneca and Sanofi Pasteur; ClinicalTrials.gov number, NCT02878330.).