Abstract

Background: Clesrovimab is a long-acting investigational monoclonal antibody against site IV of the respiratory syncytial virus (RSV) fusion protein. Data regarding the safety and efficacy of clesrovimab in healthy infants are needed.

Methods: We randomly assigned healthy preterm and full-term infants entering their first RSV season in a 2:1 ratio to receive one intramuscular 105-mg dose of clesrovimab or placebo. The primary efficacy end point was RSV-associated medically attended lower respiratory infection (including at least one indicator of lower respiratory infection or disease severity) through 150 days after injection. A key secondary efficacy end point was RSV-associated hospitalization during the same period.

Results: A total of 3614 infants received an injection: 2412 infants received clesrovimab, and 1202 infants received placebo. Through day 150 after injection, RSV-associated medically attended lower respiratory infection occurred in 60 of 2398 infants in the clesrovimab group (incidence rate over 5-month period, 2.6%) and in 74 of 1201 infants in the placebo group (incidence rate over 5-month period, 6.5%), for an efficacy of 60.4% (95% confidence interval [CI], 44.1 to 71.9; P<0.001). RSV-associated hospitalization within 150 days was reported in 9 of 2398 infants in the clesrovimab group and in 28 of 1201 infants in the placebo group, for an efficacy of 84.2% (95% CI, 66.6 to 92.6; P<0.001). Serious adverse events were reported in 278 of 2409 infants (11.5%) in the clesrovimab group and 149 of 1202 infants (12.4%) in the placebo group.

Conclusions: In healthy preterm and full-term infants, a single dose of clesrovimab reduced the incidence of RSV-associated medically attended lower respiratory infection and RSV-associated hospitalization, with a safety profile similar to that of placebo. (Funded by Merck Sharp and Dohme; CLEVER ClinicalTrials.gov number, NCT04767373.).