Clesrovimab's second RSV season adds safety data, not comparative efficacy

In 997 high-risk infants, first-season adverse events were numerically similar with clesrovimab and palivizumab. The 276-child second-season cohort had no concurrent comparator, so comparative efficacy was not tested.

The hot take

There was no comparator arm in season 2 of this study.

The end-of-study SMART report gives useful safety data for clesrovimab in infants at increased risk of severe respiratory syncytial virus disease, including a second season of dosing. It does not show that second-season clesrovimab prevents more RSV illness than palivizumab, placebo, or no prophylaxis because every child who continued into season 2 received clesrovimab.

What the trial did

SMART was a phase 3, randomized, partially masked, active-controlled trial conducted at 110 sites across 27 countries and territories from November 2021 to November 2025. Of 1003 randomized infants, 997 were treated: 498 received clesrovimab and 499 received palivizumab. The primary objective was safety and tolerability. The season 1 safety analysis included 497 clesrovimab recipients and 499 palivizumab recipients because one participant who received a mixed regimen was excluded.