Merck and Moderna Phase 3 mRNA Cancer Vaccine Win Lifts Pfizer Stock While Raising Competitive Stakes

Pfizer shares rose about 1% to $27.50 on Wednesday, August 19, bringing the stock's year-to-date gain to roughly 15%, as broad enthusiasm over a landmark cancer vaccine trial lifted the oncology sector. Moderna and Merck announced that intismeran, their personalized mRNA cancer vaccine, combined with Merck's Keytruda, met its primary endpoint in a Phase 3 adjuvant melanoma trial, marking the first randomized late-stage validation of a personalized neoantigen cancer vaccine. Moderna shares surged as much as 93% to 150% on the news, while Merck gained about 7%.

Pfizer has no direct role in the trial but benefits from renewed investor enthusiasm around oncology combinations. The result significantly raises the competitive bar for Pfizer's own cancer vaccine ambitions, as the Moderna-Merck program now has Phase 3 proof of concept that rivals in the space will need to match. Options markets also flagged elevated bearish positioning in Pfizer, with unusual put activity noted in the September 25 and September 27 strikes.

Separately, Pfizer was among the nominees in the broader field for the 2026 International Prix Galien, the life sciences industry's top global innovation award.