Good news for melanoma patients turned into a rough day for investors, and the reason why is stranger than it sounds.
Moderna and Merck released Phase 3 cancer vaccine results on October 22, 2026, showing their personalized melanoma treatment cut the risk of recurrence or death by 42% when paired with Keytruda. Despite the strong numbers, Moderna shares sank as much as 25% after trial details leaked online before the companies could present them.
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Moderna Merck Cancer Vaccine Results Show Real Benefit For Patients
The trial tested intismeran autogene, a cancer vaccine built from each patient's own tumor, combined with Merck's immunotherapy drug Keytruda. More than 1,100 patients with high-risk melanoma took part, comparing the combo treatment against Keytruda alone. After a median follow-up of 24.6 months, the vaccine-plus-Keytruda group saw a 42% lower risk of their cancer coming back or causing death, compared to Keytruda by itself.
The vaccine also reduced the risk of the cancer spreading to distant parts of the body, or death, by 38%. At the two-year mark, 82.2% of patients on the combination treatment had no sign of cancer returning, versus 70% of those on Keytruda alone. A similar gap showed up in distant spread of the disease, with 88.5% of combination patients staying free of it compared to 80.2% on Keytruda alone. These are the kind of numbers doctors wait years to see in melanoma research.
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Moderna Stock Falls Despite Strong Cancer Vaccine Results
Here's where things get messy. The companies had scheduled a formal presentation of this data at the European Society for Medical Oncology meeting in Madrid over the weekend. But an oncology news website published details early, on the morning of October 22, and the numbers spread across social media before Moderna and Merck could control the story.
Moderna shares dropped as much as 25% during the trading day and closed down roughly 17.6% at $202.63. Merck, a much bigger and more diversified company, fell a smaller amount, about 1.8%, closing near $147.24. One analyst called it a classic “sell-the-news” reaction, meaning investors had already bet so heavily on a great outcome that even genuinely good results felt like a letdown.
That reaction makes more sense once you look at what happened before the leak. Moderna's stock had already surged nearly fourfold since August, when the companies first shared early interim results pointing toward a benefit. That rally baked sky-high expectations into the stock price, leaving almost no room for anything less than a jaw-dropping surprise. When solid, but not shocking, numbers finally arrived, traders sold.
Why A Scientific Data Leak Rattled Wall Street
Leaks like this are rare in major drug trials, and they carry real consequences. Moderna and Merck had planned a controlled rollout of this data, giving doctors and investors a chance to digest the full picture at once, alongside expert commentary at the medical conference. Instead, partial information spread in fragments online, stripped of context, before the companies released the complete results later that same day.
Without the full picture, early readers of the leaked data may have misjudged how strong or weak the findings actually were. That kind of confusion can spook markets fast, especially when a stock has already climbed as sharply as Moderna's did. The companies did not offer a public explanation of how the leak happened or who was responsible, based on available reporting. Their public statements instead focused on the trial numbers themselves and what they could mean for future melanoma care.
What This Means For Cancer Treatment And Company Strategy
Beyond the stock swings, this trial matters because it could change how melanoma is treated after surgery. Checkpoint drugs like Keytruda have helped many patients for years, but recurrence remains a real threat for people with high-risk disease. A personalized vaccine built from a patient's own tumor mutations, designed to train the immune system to hunt down leftover cancer cells, represents a genuinely different approach.
For Moderna, this program carries extra weight. The company built its reputation and much of its revenue on COVID-19 vaccines, and investors have been pushing hard for proof that Moderna can succeed in other areas of medicine. A win in melanoma, especially one built on mRNA technology, would give the company a new growth story beyond its pandemic-era business. For Merck, pairing Keytruda with a personalized vaccine could extend the drug's usefulness and strengthen its position in a competitive cancer treatment market.
What Comes Next For Patients, Doctors, And Investors
The trial results still need to go through formal presentation and review at the medical conference, where oncologists will scrutinize safety data, side effects, and how long the benefits actually last. Regulators have not yet approved intismeran autogene, and the available reporting does not point to a specific filing date or approval timeline.
Going forward, close watchers will be tracking whether the benefit holds up with longer follow-up, whether Moderna can manufacture personalized vaccines at scale without major delays or added costs, and whether the approach can expand beyond melanoma into other cancers. For now, the science looks promising. The stock market reaction, though, is a reminder that even a genuine medical breakthrough can disappoint traders who expected something even bigger.
