New phase 3 trial data bolster Merck’s clesrovimab as a promising option to protect children under 2 at high risk of severe respiratory syncytial virus (RSV) illness entering their second season, with FDA review underway for expanded approval.
Merck said new late-stage trial data support clesrovimab as a possible option for protecting children at high risk of severe respiratory syncytial virus illness as they enter a second RSV season. The company’s update adds to earlier evidence behind the monoclonal antibody, which the US Food and Drug Administration approved last year for infants during their first season.
In the phase 3 SMART trial, researchers followed children who had already completed one RSV season and then received a second dose before the next season began. According to results presented by Merck, the safety profile in the second season was broadly in line with what had been seen previously, and drug levels remained in the range associated with protection in earlier studies. The company said the findings strengthen the case for use in younger children who remain vulnerable because of conditions such as chronic lung disease or congenital heart disease.
The latest data presented at a respiratory virus conference in Rome covered children under age 2 who were still considered at increased risk. Across the second season, the study reported medically attended lower respiratory tract infections and RSV-related hospitalisations, with no clear difference in RSV outcomes between children who had received clesrovimab and those who had received palivizumab in the first season. Safety events were common but mostly mild, with irritability, sleepiness and reduced appetite among the most frequently reported.
Researchers also said there were no cases of anaphylaxis in either season. One child died during the second season and another after the study ended, but neither death was judged related to treatment. Merck said the data support its request to broaden clesrovimab’s approval to children up to age 2 who are entering their second season at high risk of severe RSV disease. The FDA has set a target decision date of March 22, 2027.
The application would place clesrovimab alongside nirsevimab, the longer-acting antibody now used to protect infants entering their first RSV season and selected older infants entering a second season. Unlike nirsevimab, which is dosed by weight, clesrovimab is given as a fixed dose. Real-world evidence has already shown substantial declines in RSV disease after use of nirsevimab and maternal vaccination, but clesrovimab has only been available for one season and has not yet been assessed in routine practice.
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