Moderna shares plunge 17% as leaked melanoma data confirms 42% risk reduction

Key points

• Intismeran combined with Keytruda reduced the risk of melanoma recurrence or death by 42% after two years, compared to Keytruda alone.
• Moderna’s stock fell 17% and Merck’s dropped 4% following the leak, despite the data confirming the drug’s efficacy.
• The trial included 1,137 patients with stage IIB–IV melanoma, showing 82% recurrence-free survival for the vaccine group versus 70% for the control.
• The results were leaked before the official presentation at the European Society of Medical Oncology in Madrid, prompting immediate release by the companies.
• Side effects included higher rates of fever and injection-site pain, with 7% of vaccine recipients discontinuing treatment due to adverse events.

Background

In August 2026, Moderna and Merck announced that their Phase 3 trial for intismeran had met its primary endpoints, triggering a 177% surge in Moderna’s stock. The detailed data was scheduled for release at the ESMO conference in Madrid on October 24, but a leak on October 22 forced the companies to disclose the results early. Previous Phase 2 trials had shown a 49% risk reduction, setting high expectations for the larger Phase 3 study.

How outlets are covering it

Yahoo Finance emphasized the market volatility, noting that Moderna’s stock had nearly quadrupled since August, making the 17% drop its worst since the initial rally. Ars Technica focused on the clinical significance, highlighting that the 42% reduction represents a major advance in melanoma treatment and the first Phase 3 success for personalized mRNA vaccines. BioPharma Dive analyzed the investor reaction, noting that while the data met the 35% clinical threshold, it fell short of the 44-50% expected by Wall Street, leading to a 'sell-the-news' reaction. Pioneer Press highlighted the broader implications for cancer treatment, with experts calling the results a 'new frontier' for oncology.

Why it matters

The results confirm that personalized mRNA vaccines can effectively reduce cancer recurrence, validating the technology’s potential beyond COVID-19. However, the market reaction underscores the high expectations for biotech stocks and the challenges of commercializing new therapies. The data also sets a new benchmark for adjuvant melanoma treatment, potentially influencing future drug approvals and clinical trials.

What to watch

Moderna and Merck are engaging with regulatory authorities to seek approval for intismeran. Additional studies are underway for lung, bladder, and kidney cancers, with interim results expected in late 2026 or early 2027. The full data will be presented at the ESMO conference in Madrid on October 24, with long-term outcomes to be tracked in ongoing trials.