Merck’s Enflonsia FDA Approval Spurs Sanofi to Accelerate Beyfortus Supply Ahead of 2025-26 RSV Season

Merck’s new RSV monoclonal antibody, clesrovimab (marketed as Enflonsia), received FDA approval on June 9, 2025, for protecting infants under 12 months during their first RSV season. It is currently not approved for high-risk infants in their second RSV season, a distinction from Sanofi's Beyfortus23.

Sanofi, in partnership with AstraZeneca, is ramping up global shipments of Beyfortus (nirsevimab) to ensure broad availability ahead of the 2025-2026 RSV season. Shipments will begin early in Q3 2025, matching last year’s total U.S. doses and reflecting surging demand4.

Beyfortus is now supported by robust real-world efficacy data, showing over 80% effectiveness at preventing RSV across term and preterm infants, with positive health system impacts in various geographies including Spain and Chile5.

Sanofi and AstraZeneca have increased production capacity for Beyfortus threefold and doubled the number of manufacturing sites since its 2023 launch to support anticipated demand spikes4.

Unlike Beyfortus, Merck's Enflonsia is still undergoing comparative studies against Synagis for potential approval in high-risk infants’ second RSV season2.

The CDC’s vaccine advisory committee, expected to guide Enflonsia use, is undergoing a complete membership overhaul, raising questions about timely immunization policy recommendations for the new product2.

Sources:

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

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

4. https://www.sanofi.com/en/media-room/press-releases/2025/2025-06-09-05-00-00-3095598

5. https://www.ainvest.com/news/beyfortus-sanofi-rsv-breakthrough-cementing-dominance-global-demand-surge-2506/