Merck Wins FDA Approval for RSV Antibody Drug, Intensifying Market Competition

The FDA approved Merck’s monoclonal antibody clesrovimab, to be marketed as Enflonsia, for the prevention of respiratory syncytial virus (RSV) in newborns and infants entering their first RSV season12345.

Enflonsia provides a single 105 mg dose for all infants, regardless of weight, aiming to protect through the full 5-month RSV season1.

Enflonsia joins Sanofi and AstraZeneca’s Beyfortus (nirsevimab-alip), which was FDA-approved in 2023, making this a more competitive market124.

Phase 2b/3 clinical trials showed Enflonsia reduced RSV-related medically attended lower respiratory infections by 60.5% and RSV hospitalizations by 84.3% over 5 months compared to placebo14.

An additional Phase 3 trial showed safety and efficacy compared to palivizumab, an older RSV antibody for high-risk infants14.

Merck plans to make Enflonsia available for the 2025-26 RSV season45.

RSV remains a leading cause of hospitalization and death in infants, underlining the importance of effective prevention options4.

The approval increases competition against Sanofi and AstraZeneca in the expanding RSV prevention market24.

Sources:

1. https://www.cidrap.umn.edu/respiratory-syncytial-virus-rsv/fda-approves-merck-rsv-monoclonal-antibody-shot-infants

2. https://www.biospace.com/fda/merck-wins-fda-nod-for-rsv-antibody-tees-up-challenge-to-sanofi-astrazeneca

3. https://www.statnews.com/2025/06/09/fda-approves-rsv-monoclonal-antibody-from-merck-for-infants/

4. https://www.contagionlive.com/view/merck-s-clesrovimab-receives-fda-approval-to-protect-infants-from-rsv-during-first-season

5. https://www.clinicaltrialsarena.com/analyst-comment/mercks-enflonsia-receives-fda-approval-rsv-prophylaxis/