Merck Hits Best Single-Day Gain Since 2009 After Phase 3 Melanoma Vaccine Trial Success With Moderna

Merck surged more than 12% on August 19, its best intraday gain since 2009, after the company and Moderna reported positive topline results from the Phase 3 INTerpath-001 trial of their personalized mRNA cancer vaccine, intismeran. The trial, which tested intismeran combined with Merck's checkpoint inhibitor Keytruda, showed the therapy slowed melanoma recurrence and spread in late-stage testing. The study was halted at its first interim analysis after crossing the positive threshold, and Benzinga noted it marks the first-ever positive Phase 3 result for an mRNA-based personalized neoantigen cancer therapy.

Moderna shares soared more than 50% on the news, while Personalis, which helps sequence tumors to design neoantigen vaccines, rose roughly 6.3% in pre-market trading. By late in the session, Merck had climbed to all-time highs and ranked as the second-best Dow Jones Industrial Average stock year-to-date, within a percentage point of the top spot. Separately, a Seeking Alpha analyst downgraded MRK from Strong Buy to Hold, citing the stock's 72% run since their last bullish call, a valuation of roughly 14 times 15-month forward earnings, and limited margin for error in lofty fiscal 2027 earnings expectations.