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Updated: Jan 17, 2026
An Improved and High Throughput Respiratory Syncytial Virus RSV Micro-neutralization Assay
Published on: January 26, 2019
Clesrovimab for Prevention of RSV Disease in Healthy Infants
Heather J Zar1, Eric A F Simões2, Shabir A Madhi3
1Department of Pediatrics and Child Health and Medical Research Council Unit on Child and Adolescent Health, University of Cape Town, Cape Town, South Africa.
Insights
A single dose of clesrovimab significantly reduced medically attended lower respiratory infections and hospitalizations in infants. This investigational monoclonal antibody demonstrated a safety profile comparable to placebo in healthy infants.
Area of Science:
- Immunology
- Pediatrics
- Virology
Background:
- Clesrovimab is an investigational long-acting monoclonal antibody targeting the respiratory syncytial virus (RSV) fusion protein.
- Limited data exist on the safety and efficacy of clesrovimab in healthy infants.
Purpose of the Study:
- To evaluate the safety and efficacy of a single intramuscular dose of clesrovimab in healthy preterm and full-term infants during their first RSV season.
Main Methods:
- A randomized trial assigned 3614 healthy infants (2:1 ratio) to receive either clesrovimab or placebo.
- The primary endpoint was RSV-associated medically attended lower respiratory infection through 150 days post-injection.
- A secondary endpoint included RSV-associated hospitalization within the same period.
Main Results:
- Clesrovimab demonstrated 60.4% efficacy against RSV-associated medically attended lower respiratory infection (2.6% vs. 6.5% incidence).
- Efficacy against RSV-associated hospitalization was 84.2% (0.4% vs. 1.3% incidence).
- Serious adverse events were similar between the clesrovimab (11.5%) and placebo (12.4%) groups.
Conclusions:
- A single dose of clesrovimab effectively reduced RSV-related lower respiratory infections and hospitalizations in healthy infants.
- The safety profile of clesrovimab was comparable to placebo in this infant population.
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.).
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