VXB-251 (Vicebio)

VXB-251, also designated **SP0341** by Sanofi, is an investigational trivalent subunit vaccine originally developed by Vicebio for prevention of lower respiratory tract disease caused by respiratory syncytial virus, human metapneumovirus, and human parainfluenza virus 3 in older adults. The vaccine uses Vicebio's Molecular Clamp technology, which stabilizes viral proteins in their native conformation to support recognition by the immune system and induction of protective antiviral immune responses. Sanofi acquired Vicebio in December 2025 and is advancing the candidate in a Phase 1 trial in older adults. ([sanofi.com](https://www.sanofi.com/assets/dotcom/content-app/events/quaterly-results/2026/2026-q2-2026-results/20260730-Sanofi-Q2-2026-RnD-Appendices.xlsm))