Merck's New Therapy Only Reduces Infant RSV Infections by 60.4%, Hospitalization Rate Decline of 84.2% Raises Safety Concerns

Merck announced Thursday that its investigational antibody treatment, clesrovimab, demonstrated a 60.4 percent effectiveness in reducing infections associated with a specific type of respiratory syncytial virus (RSV) in infants. This seasonal injection regimen successfully achieved its primary goal in the mid to late phase of the trial, significantly reducing the incidence of medically care-required lower respiratory tract infections due to RSV in healthy infants under 1 year of age during the first RSV season.

Merck further noted that clesrovimab also significantly reduced RSV-related hospitalizations by as much as 84.2% compared to placebo. The rate of side effects caused by the drug during treatment was comparable to that of the placebo group, and there were no deaths directly related to treatment or RSV during the study period. Merck previously announced in July that the vaccine had met the main goal of the study, but did not disclose details at the time.

If approved, clesrovimab will compete in the U.S. market with Sanofi's (SASY.PA) and Astrazeneca's (AZN.L) Beyfortus, also an antibody therapy that prevents RSV infections in infants and children and requires a quarterly injection. In a separate late-stage study, Beyfortus was able to reduce the incidence of lower respiratory infections in infants requiring medical care by 74.5%.

In addition, Synagis from Swedish orphan drug company Biovitrum (SOBIV.ST) is available as a treatment to prevent RSV, requiring monthly injections. Merck reported on Thursday that clesrovimab was on par with Synagis in terms of safety, a conclusion based on interim results from another late-stage study.

Notably, Pfizer's RSV vaccine Abrysvo is already approved for use in pregnant women and is designed to prevent RSV infection in babies during the first six months of life. RSV often causes cold-like symptoms and is one of the leading causes of pneumonia and death in infants and the elderly. In the United States, the virus causes about 177,000 hospitalizations and 14,000 deaths each year.