Moderna Stock Plunges 25% as Cancer Vaccine Data Leak Erases $24 Billion
Moderna shares plunged more than 25% on Thursday, October 22, 2026, after clinical trial results for its experimental cancer vaccine surfaced ahead of schedule. The selloff erased approximately $24 billion in market value, reversing part of the biotechnology company’s extraordinary rally. Shares fell to around $183.88 during Thursday’s trading, while development partner Merck declined nearly 1%.
The leaked Phase 3 results concerned intismeran, Moderna’s personalized mRNA cancer vaccine developed alongside Merck. In a trial involving more than 1,100 melanoma patients, intismeran combined with Merck’s Keytruda reduced cancer recurrence or death risk by 42%. The combination also lowered the risk of distant cancer spread or death by 38% compared with Keytruda alone. Moderna and Merck subsequently released detailed findings after the information appeared prematurely on an oncology website.
Despite encouraging clinical results, analysts attributed the sharp decline to elevated investor expectations. Moderna shares had surged more than 500% earlier in 2026, leaving little room for disappointment. Analysts at UBS and William Blair characterized the reaction as profit-taking rather than evidence of weak clinical performance. The companies plan to present their findings at the European Society for Medical Oncology meeting in Madrid on October 24.
Why it matters
Moderna's dramatic decline highlights the valuation risks facing biotechnology investors, even when clinical trials deliver encouraging results. Future regulatory decisions and evidence of effectiveness across additional cancers could influence the company's long-term growth prospects.
