Moderna Inc. shares declined roughly 17% on Thursday, leading S&P 500 losers, as investors locked in profits after the stock more than tripled on Wednesday. The prior-day surge followed the announcement that a personalized mRNA-based cancer vaccine, developed in partnership with Merck & Co., met key endpoints in a large Phase 3 trial for high-risk melanoma.
The study enrolled more than 1,100 patients with stage IIB–IV melanoma whose tumors had been surgically removed. The combination of the individualized vaccine (intismeran) plus Merck’s Keytruda demonstrated statistically significant and clinically meaningful improvements in both recurrence-free survival, the primary endpoint, and distant metastasis-free survival compared with Keytruda alone. Detailed numerical data were not released and will be presented at a future medical meeting.
The results mark the first positive late-stage readout for a personalized mRNA cancer vaccine and validate years of research into neoantigen-based therapies. Analysts have projected potential multi-billion-dollar peak sales in melanoma alone, with additional upside if the approach expands into other tumor types. Merck shares, which rose more than 12% on Wednesday, eased modestly in the follow-through session.
Despite the pullback, the magnitude of Wednesday’s move reflected the transformative potential for Moderna’s oncology franchise at a time when COVID-related vaccine revenues have declined. For portfolio positioning, the event highlights the binary nature of clinical catalysts in biotechnology and the tendency for extreme short-term volatility. Longer-term investors will focus on regulatory filing timelines, commercial manufacturing readiness, and the competitive landscape for next-generation immuno-oncology combinations.