Respiratory Syncytial Virus Prevention with Clesrovimab
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Reviewed & Translated by Dat Tien Nguyen, B.A, ScM.
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Posted on October 24th, 2025
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Respiratory syncytial virus infection is the leading cause of infant hospitalization, with the disease severity highest for infants below 6 months old. Many studies had found monoclonal antibodies such as nirsevimab and palivizumab to be an effective preventive measure for RSV infection in infants. With funding from Merck, a study was conducted to investigate the usage of clesrovimab to prevent RSV infection in young children.
The clinical study included 3614 infants at an average age of 3.7 months old. 82.5% of these children were born full term with a gestational age of more than 35 weeks, and the remaining were born late-preterm with the gestational age between 29 and 34 weeks. These infants were randomly assigned to receive an intramuscular injection of either placebo or 105 mg of clesrovimab.
Similar to frequently used agents such as nirsevimab and palivizumab, clesrovimab is also a monoclonal antibody that targets the fusion protein that the RSV virus used to enter and infect cells. Palivizumab target site II of the fusion (F) protein, nirsevimab target site Ø, and clesrovimab bind to site IV. This target is present on both the prefusion and postfusion conformation of the F protein; whereas, site Ø is only present on the prefusion form. Additionally, site IV is very conserved; therefore, there is a lower risk of resistance.
Surveillance at 180 days after injection found that nirsevimab was 90% effective at preventing hospitalization due to severe lower respiratory infection and almost 100% effective at preventing RSV-associated hospitalization. In the 7 patients who were injected with clesrovimab that develop severe infection, the RSV possesses a G446E mutation that might have interfered with antibody binding. Future studies should perform direct comparison between clesrovimab and other monoclonal antibodies targeting RSV.
The clinical study included 3614 infants at an average age of 3.7 months old. 82.5% of these children were born full term with a gestational age of more than 35 weeks, and the remaining were born late-preterm with the gestational age between 29 and 34 weeks. These infants were randomly assigned to receive an intramuscular injection of either placebo or 105 mg of clesrovimab.
Similar to frequently used agents such as nirsevimab and palivizumab, clesrovimab is also a monoclonal antibody that targets the fusion protein that the RSV virus used to enter and infect cells. Palivizumab target site II of the fusion (F) protein, nirsevimab target site Ø, and clesrovimab bind to site IV. This target is present on both the prefusion and postfusion conformation of the F protein; whereas, site Ø is only present on the prefusion form. Additionally, site IV is very conserved; therefore, there is a lower risk of resistance.
Surveillance at 180 days after injection found that nirsevimab was 90% effective at preventing hospitalization due to severe lower respiratory infection and almost 100% effective at preventing RSV-associated hospitalization. In the 7 patients who were injected with clesrovimab that develop severe infection, the RSV possesses a G446E mutation that might have interfered with antibody binding. Future studies should perform direct comparison between clesrovimab and other monoclonal antibodies targeting RSV.