Clesrovimab protects infants against RSV-associated infections, hospitalizations

A single dose of clesrovimab, a long-acting monoclonal antibody, significantly reduced the incidence of respiratory syncytial virus (RSV)-associated medically attended lower respiratory infections (LRIs) and hospitalizations in healthy infants, according to the phase IIb–III CLEVER trial.

“RSV is a leading cause of infant hospitalization in high-income countries and a frequent cause of infant deaths in low- and middle-income countries, … [with the highest] burden of severe RSV infection during the first 6 months of life, when prevention is needed,” said the researchers.

Hence, the researchers conducted a study involving 3,614 healthy preterm and full-term infants (median age 3.1 months) entering their first RSV season. Participants were randomized in a 2:1 ratio to receive a single intramuscular dose of clesrovimab 105 mg (n=2,412) or a placebo (n=1,202). [N Engl J Med 2025;doi:10.1056/NEJMoa2502984]

During the 150-day post-injection period, 60 infants in the clesrovimab group experienced RSV-associated medically attended LRI compared with 74 infants in the placebo group, resulting in an incidence rate of 2.6 percent vs 6.5 percent. This corresponds to an efficacy of 60.4 percent (p<0.001) for clesrovimab in preventing RSV-associated LRIs.

RSV-associated hospitalizations during the same period were also significantly reduced in the clesrovimab group than in the placebo group (9 vs 28 infants), resulting in an efficacy rate of 84.2 percent (p<0.001).

In terms of safety, the rates of adverse events (AEs) were similar between the clesrovimab and placebo arms (75.4 percent vs 76.4 percent), as were serious AEs (11.5 percent vs 12.4 percent).

The incidence of deaths was also comparable (0.3 percent for clesrovimab and 0.2 percent for placebo), with none of the deaths considered related to clesrovimab or placebo or attributed to RSV, noted the researchers.

The most common AEs reported in the clesrovimab group were injection-site reactions (9.3 percent) and irritability (18.7 percent), which were similar to those observed in the placebo group (9.7 percent and 19.7 percent, respectively).

According to the researchers, the AE profile of clesrovimab was similar to that of placebo, with the most common AEs being conditions commonly observed in infants in the trial age group.

Overall, the findings demonstrate that “clesrovimab, administered as one 105-mg dose before or during the first RSV season, provides protection against mild, moderate, and severe RSV disease in healthy preterm and full-term infants,” said the researchers.

Furthermore, the significant reduction in RSV-associated hospitalizations highlights “a potential additional public health benefit, as lower respiratory infections remain a predominant cause of infant hospitalization and death worldwide,” they added.