Pfizer's mRNA flu shot outperforms Sanofi’s Fluzone by 34.5% in late-stage trial

Pfizer’s mRNA flu vaccine showed 34.5% greater efficacy than Sanofi’s Fluzone in a phase 3 trial, especially against the more severe influenza A strains.

Why it matters: This marks a significant step toward modernizing flu prevention. If approved, it would be the first mRNA-based flu vaccine, potentially offering better protection and faster adaptability to virus mutations.

Backstory: The 18,000-participant study compared Pfizer’s mRNA shot to Sanofi’s conventional vaccine. It met efficacy criteria for influenza A (the most common and most severe) but not B, mirroring early challenges faced by Moderna. Moderna later improved its formulation to target B strains.

Zoom in: Of 9,225 people who received Pfizer’s mRNA vaccine, only 57 developed a flu case, while 87 of 9,251 had a flu case with Sanofi’s Fluzone shot (control group). Side effects like fatigue and injection-site pain were more common with the mRNA shot, but severe events were rare and similar between groups.

Big picture: mRNA technology, which proved its worth during the COVID-19 pandemic, could transform flu vaccination with faster production and updates. Moderna plans to seek global approvals for its flu shot by January 2026, and Pfizer is on a similar track.