Moderna Crashes 25% After Phase 3 Melanoma Vaccine Data Leaked on Social Media Before ESMO Presentation

Moderna shares plunged as much as 26% on Thursday, on track for the stock's worst single-day decline since its 2018 IPO, after closely guarded Phase 3 clinical trial data for its experimental personalized cancer vaccine were leaked on social media and the medical website Oncology Central before being pulled down. The data, from the INTerpath-001 trial, had been scheduled for formal presentation at the ESMO 2026 Presidential Symposium on Saturday. The drop erased roughly $12 billion in market value and came just one day after the stock had hit its highest level in nearly five years.

The leaked figures showed that intismeran autogene, the personalized mRNA vaccine co-developed by Moderna and Merck and administered alongside Merck's blockbuster immunotherapy Keytruda, reduced the risk of melanoma recurrence or death by 42% compared with Keytruda alone. That result disappointed investors who had been expecting a stronger readout: the earlier Phase 2b trial had reported a 49% reduction. The leaked data remain unverified ahead of the official presentation, and Merck and Moderna subsequently issued a statement confirming positive recurrence-free survival and distant metastasis-free survival results from the trial without specifying the figures.

Merck confirmed the data breach and its own shares fell roughly 3.1% to 3.9%. BioNTech, a competing developer in the mRNA cancer vaccine space, also dipped, while Pfizer edged modestly higher. Options market analysis pointed to a large negative gamma wall at $200, a level traders said Moderna would need to reclaim to stabilize, with smaller support levels noted near $187.50 and $180. A law firm announced an investigation into potential securities fraud claims on behalf of investors, citing the circumstances of the October 22 disclosure.

Analysts and oncologists had flagged two central questions heading into the weekend presentation: the precise hazard ratio for the primary endpoint, and whether the results would suggest the vaccine could be effective across cancer types beyond melanoma. The leaked partial data answered the first question but left the broader applicability question unresolved ahead of Saturday's full readout.