Clesrovimab is a single-dose extended half-life monoclonal antibody developed as a passive immunisation for the prevention of RSV. Credit: Sergiy Palamarchuk/Shutterstock.

MSD (Merck & Co) is planning to get its RSV vaccine, clesrovimab, approved and launched before the next RSV season in 2025–26, after reporting positive results from a Phase IIb/III trial.

The company first reported that the Phase IIb/III MK-1654-004 trial (NCT04767373) evaluating clesrovimab as a preventative treatment for RSV in infants met its primary endpoint in July. MSD has now presented data showing that the study evaluating the therapy met the primary endpoint by reducing the incidence of RSV-associated medically attended lower respiratory infections (MALRI) requiring at least one indicator of lower respiratory infections by 60% through the first five months after dosing, compared with the placebo.

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The results were presented at IDWeek 2024 from 16 to 19 October in Los Angeles. MSD noted that the company is “continuing to discuss these data with health authorities around the world with the goal of making clesrovimab available for infants as early as the 2025–26 RSV season”.

If approved, clesrovimab’s main competitor will be AstraZeneca and Sanofi’s Beyfortus (nirsevimab), which is approved in the US for preventing RSV lower respiratory tract disease (LRTD) in newborns and infants. GlobalData estimates that Beyfortus will eclipse clesrovimab sales due to its first-to-market advantage, with the former raking in $2.5bn in 2030 compared with $408m for the latter over the same period.

GlobalData is the parent company of Clinical Trials Arena.

Clesrovimab is a single-dose monoclonal antibody with an extended half-life developed as a passive immunisation for the prevention of RSV. The placebo-controlled Phase IIb/III MK-1654-004 trial enrolled 3,632 infants under one year old. Clesrovimab met its secondary endpoint by reducing RSV-associated hospitalisations over five months by 84.2% compared with placebo. The trial also met its tertiary endpoint of reducing RSV-associated LRTD hospitalisations by 90.9% versus placebo through five months.

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MSD also presented data from an interim analysis from the Phase III MK-1654-007 trial (NCT04938830) that compared clesrovimab to Sobi’s RSV monoclonal antibody vaccine Synagis (palivizumab). The study is evaluated the vaccines in infants and children at increased risk for severe RSV disease.

At the time of interim analysis, 901 participants were enrolled in the trial. The results showed that clesrovimab had a comparable safety profile to Synagis. Additionally, the incidence rates of RSV-associated MALRI requiring at least one indicator of lower respiratory infections and RSV-associated hospitalisations were comparable between the two groups – 3.6% and 1.3% in clesrovimab and 3% and 1.5% in Synagis groups, respectively.