One of the most closely watched experiments in cancer medicine hit the public in the least orderly way possible on Thursday, when detailed results from a late-stage trial of Moderna’s personalised melanoma vaccine leaked online ahead of schedule — and the market’s reaction wiped around $24 billion from the company’s value.
The data, for a treatment called intismeran developed jointly by Moderna and Merck, appeared on the oncology website Oncology Central before its planned presentation at the ESMO cancer conference in Madrid on Saturday. The leak forced the conference organisers to lift their embargo, and the two companies released the full results themselves.
On the numbers, the trial was a success. More than 1,100 high-risk melanoma patients who had undergone surgery received either intismeran plus Merck’s Keytruda or Keytruda alone. The combination reduced the risk of cancer returning or of death by 42%, and cut the risk of distant metastasis or death by 38%. About 82% of patients who received the vaccine were alive without their melanoma returning at two years, compared with roughly 70% on Keytruda alone.
Dean Li, president of Merck Research Laboratories, called the study “one of the most significant advances in melanoma treatment since the advent of immunotherapy.”
Analysts agreed the science was strong. UBS’s Michael Yee described the data as “solid”; William Blair’s Myles Minter went further, calling it “great.”
The share price told a different story. Moderna stock sank as much as 25.2% to $183.88 during Thursday’s session — its steepest one-day fall on record — before closing 17.6% lower. Merck shares slipped about 1%.
The rout, described by analysts as a classic “sell-the-news” reaction, followed a months-long rally that had lifted Moderna’s stock more than 500% this year as investors piled in ahead of the trial readout. With expectations set so high, strong-but-not-miraculous data gave holders an excuse to take profits.
Overall survival data from the trial is not yet mature, though the companies said it shows a “favourable trend.” Moderna and Merck said they are engaging with regulators and plan to pursue approval. The formal presentation of the results is still scheduled for Saturday in Madrid.
Sources: Reuters (https://www.reuters.com/business/healthcare-pharmaceuticals/moderna-sheds-about-24-billion-cancer-vaccine-data-triggers-sell-the-news-2026-10-22/) and MarketWatch (https://www.marketwatch.com/story/modernas-stock-tumbles-after-reported-leak-of-data-for-cancer-vaccine-dcc897c1).
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