TOPLINE:

A single 105-mg dose of clesrovimab was efficacious in preventing respiratory syncytial virus (RSV)-associated medically attended lower respiratory infections and was well tolerated in healthy premature or full-term infants, according to a recent study.

METHODOLOGY:

• Researchers conducted a phase 2b-3 study across 22 countries to assess the efficacy and safety of clesrovimab in healthy early premature or moderately premature infants (gestational age, 29-34 weeks 6 days) or late premature or full-term infants (gestational age, ≥ 35 weeks) entering their first RSV season.
• Researchers randomly assigned 3614 infants (median age at enrollment, 3.1 months) to receive either a single intramuscular 105-mg dose of clesrovimab (n = 2412) or placebo (n = 1202).
• The primary efficacy endpoint was RSV-associated medically attended lower respiratory infection showing more than one sign of infection or disease severity through 150 days post-injection. Safety was also assessed as a primary endpoint.
• Secondary endpoints assessed RSV-associated hospitalization through 150 days post-injection.

TAKEAWAY:

• At 150 days, 60 vs 74 infants in the clesrovimab vs placebo groups experienced RSV-associated medically attended lower respiratory infection, reflecting 60.4% efficacy (P < .001).
• Similarly, 9 vs 28 infants in the clesrovimab vs placebo groups experienced RSV-associated hospitalization through day 150, reflecting 84.2% efficacy (P < .001).
• The number of infants who needed to be immunized to prevent one RSV infection ranged from 15 for RSV-associated acute respiratory infection to 108 for severe medically attended lower respiratory infection.
• One serious adverse event in each group related to clesrovimab (fever leading to hospitalization) and placebo (B-cell precursor acute lymphoblastic leukemia) was reported. No deaths were considered related to clesrovimab, placebo, or RSV.

IN PRACTICE:

“[The study] findings show that clesrovimab, administered as one 105-mg dose before or during the first RSV season, provided protection against mild, moderate, and severe RSV disease in healthy preterm and full-term infants,” the authors of the study wrote.

SOURCE:

The study was led by Heather Zar, MD, PhD, Department of Pediatrics and Child Health and Medical Research Council Unit on Child and Adolescent Health, University of Cape Town, Cape Town, South Africa. It was published online on September 17, 2025, in The New England Journal of Medicine.

LIMITATIONS:

No limitations were mentioned in the article.

DISCLOSURES:

The study was supported by Merck Sharp and Dohme, a subsidiary of Merck. Several authors reported being employed by the funding company and/or owning stocks or stock options in the company.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.