Moderna’s Melanoma Vaccine Data Leaks, Triggering 25% Stock Plunge Despite Clinical Success

Key points

• Intismeran autogene, paired with Merck’s Keytruda, reduced the relative risk of disease recurrence or death by 42% compared to Keytruda alone after a median of 24.6 months.
• The combination therapy also lowered the risk of cancer spreading to distant organs by 38% versus Keytruda alone.
• Moderna shares fell nearly 25% and Merck shares dropped approximately 1% following the leak, despite the data meeting clinical goals.
• The 42% reduction was slightly below the 44% seen in a smaller Phase 2 trial and the 40-50% range expected by Wall Street.
• The results are accepted for publication in the New England Journal of Medicine, and the companies are engaging with regulators for approval.

Background

In August 2026, Moderna and Merck announced that intismeran autogene met its primary endpoint in a late-stage trial, causing Moderna’s stock to quadruple. This announcement set high expectations for the detailed data presentation at the European Society for Medical Oncology (ESMO) conference in Madrid. Prior to the leak, Merck CEO Robert Davis indicated efforts to shorten the six-week manufacturing timeline for the personalized vaccine. The current data release follows a period of significant stock appreciation for Moderna, which had risen over 530% year-to-date before the leak.

How outlets are covering it

BioPharma Dive and Scientific American emphasize the clinical significance of the results, noting that the 42% reduction in recurrence risk is a major step forward for melanoma treatment, which has historically been difficult to improve upon. They highlight that while the data is positive, it falls slightly short of the 44% seen in earlier trials. CNBC and Yahoo Finance focus on the market reaction, describing the stock drop as a 'sell-the-news' event driven by the stock’s prior massive rally. Yahoo Finance notes that the 42% figure was below the 49% seen in a smaller trial, though not directly comparable. Analysts are divided: some, like Myles Minter and Tyler Van Buren, defend the data as practice-changing, while others, like Trung Huynh, argue the results are on the 'low end' of expectations and commercial success is uncertain. Karen Knudsen of the Parker Institute calls the results 'really encouraging' but notes the lack of overall survival data.

Why it matters

The results demonstrate that mRNA-based personalized cancer vaccines can effectively train the immune system to fight cancer, a technology that was considered difficult just a decade ago. The data suggests a potential new standard of care for advanced melanoma, where one-third of patients still die or see recurrence within five years despite existing treatments. However, the market reaction highlights the volatility of biotech stocks when expectations are extremely high. The success of intismeran in melanoma may pave the way for similar therapies in other cancers, though BioNTech’s recent failure in colorectal cancer shows the approach is not yet universally effective.

What to watch

Moderna and Merck are engaging with regulatory authorities to seek approval for intismeran autogene. Detailed results, including data on lower-risk patients, are scheduled for presentation at the ESMO conference in Madrid on October 24. The companies are also running additional studies in lung, bladder, and kidney cancer, with interim results from a kidney tumor trial expected later in 2026 or early 2027. Merck aims to shorten the six-week manufacturing timeline for the personalized vaccine to increase patient access.