BACKGROUND: 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.
METHODS: 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.
RESULTS: 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.
CONCLUSIONS: 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.).