August 2026

Summary:

• • • New antibody approved: Clesrovimab (Enflonsia®) received EU approval in April 2026 — the third prophylactic RSV antibody after Palivizumab and Nirsevimab.
    • Weight-independent dosing: Fixed 105 mg dose for all newborns and infants, unlike the weight-based dosing required for Nirsevimab and Palivizumab.
    • Strong efficacy: CLEVER study (Phase IIb/III) showed a 60.4% reduction in medically attended RSV infections and an 84.2% reduction in RSV-related hospitalizations over five months.
    • Good tolerability: Safety profile similar to placebo; adverse events were mostly mild-to-moderate injection-site reactions.
    • Comparable to Palivizumab in high-risk infants: SMART study showed 3.2% vs. 3.4% incidence of RSV-associated lower respiratory tract infection by day 150.
    • One dose, one season: A single dose protects for five months, the length of an RSV season; only Nirsevimab is currently approved for a second RSV season (8–19/24 months).

New RSV antibody approved: Clesrovimab as a practical alternative to Nirsevimab

With the EU approval of Clesrovimab (Enflonsia®) in April of this year, a third prophylactic antibody protecting against illness caused by RSV (respiratory syncytial virus) is now available, following Palivizumab (Synagis®) and Nirsevimab (Beyfortus®). The advantage of this newly approved active substance is its fixed dosing for newborns and infants in a single 105 mg pre-filled syringe, independent of body weight. Palivizumab and Nirsevimab, by contrast, must be dosed according to body weight (Nirsevimab: 50 mg for infants <5 kg and 100 mg for infants ≥5 kg; Palivizumab: 15 mg/kg body weight). Clesrovimab can be given to newborns and infants born during RSV season shortly after birth; for infants born outside RSV season, a single dose is administered before the start of their first RSV season.

How effective and safe Clesrovimab is in infants was demonstrated in the CLEVER study (Phase IIb/III) and the SMART study (Phase III). CLEVER enrolled healthy preterm (gestational week ≥29 to <35) and full-term (≥35 weeks) infants, who were either passively immunized with 105 mg Clesrovimab (2,412) or received saline solution (1,202). Over five months, this placebo-controlled study showed a 60.4% reduction in medically attended RSV infections and an 84.2% reduction in RSV-related hospitalizations — efficacy similar to Nirsevimab. The antibody was as well tolerated as placebo: the most common adverse events were injection-site reactions, erythema, swelling, and rash, all classified as mild to moderate. As with Nirsevimab, a single dose of Clesrovimab is sufficient.

In the SMART study, conducted in children at increased risk for severe RSV disease (preterm infants, infants with chronic lung or heart disease), Clesrovimab showed results very comparable to Palivizumab. Incidence rates of RSV-associated, medically attended lower respiratory tract infections through day 150 were 3.2% in the Clesrovimab group versus 3.4% in the Palivizumab group. In risk groups undergoing cardiac surgery with cardiopulmonary bypass during RSV season, an additional 105 mg dose is recommended once the infant is stable post-operatively, to ensure an adequate Clesrovimab serum level.

In summary, Clesrovimab is another long-acting — and in this case fixed-dose — RSV antibody, approved to protect infants for five months, the duration of an RSV season, against severe illness caused by respiratory syncytial virus. The choice between Nirsevimab and Clesrovimab depends primarily on availability, the child’s age and risk profile, and logistical factors (e.g. weight-independent dosing). In the first RSV season, both antibodies are first-line options. In the second RSV season (8–19/24 months), only Nirsevimab is currently officially approved.

Sources:

• • https://www.ema.europa.eu/en/medicines/human/EPAR/enflonsia
  • Zar HJ, et al; CLEVER (MK-1654-004) Study Group. Clesrovimab for Prevention of RSV Disease in Healthy Infants. N Engl J Med. 2025 Oct 2;393(13):1292-1303.
  • Zar HJ, et al. Clesrovimab in Infants at Increased Risk for Severe Disease During 2 RSV Seasons: A Randomized Clinical Trial. JAMA Pediatr. 2026 Jul 20:e262760.