Moderna Shares Plunge 25 Percent After Cancer Vaccine Data Leaks Early

Moderna shares fell 25 percent on Thursday, on pace for their worst day since the company went public, after detailed results from its closely watched cancer vaccine trial leaked on social media ahead of a medical conference.

The results were due to be presented this weekend at the European Society for Medical Oncology congress. The oncology news site Oncology Central published some of the data on Thursday and then pulled the article, but screenshots had already spread across X and other platforms. The society confirmed that the embargo on the study abstract had been breached and lifted the embargo on the full dataset on Thursday afternoon.

What the data showed

The vaccine, called intismeran autogene, is a personalized treatment built with messenger RNA. Moderna is developing it with Merck, and the trial tested it alongside Merck's immunotherapy Keytruda in patients with high-risk melanoma.

The leaked figures, confirmed when the full dataset was released, showed the combination cut the risk of the cancer returning or the patient dying by 42 percent compared with Keytruda alone. The measure statisticians use, the hazard ratio for recurrence-free survival, was 0.58. A second figure showed the risk of the cancer spreading to distant parts of the body fell by 38 percent.

In August, the two companies said the late-stage trial had met its main goal, and Moderna shares more than doubled on that news. The detailed numbers were the part investors had been waiting for since then.

Why the stock fell anyway

By several accounts, the numbers were good. Barron's reported the hazard ratio was better than several analysts had expected. But the market had priced in something stronger. An earlier mid-stage trial had reported a hazard ratio of 0.51 after about five years of follow-up, and Thursday's 0.58 looked like a step back from that mark.

Tyler Van Buren, an analyst at TD Cowen, called the 42 percent reduction compelling, according to The Wall Street Journal. He said the weaker spread figure fell short of the market's elevated expectations and could signal lower odds of success in cancers that have already spread widely or are harder to treat.

The sell-off was fast enough to trip safeguards. Trading in Moderna was briefly paused because of volatility, and a leveraged fund tied to the stock reported that an intraday rebalance was triggered after the shares fell 16.7 percent from Wednesday's close of $245.81. By Thursday afternoon the stock was down 25 percent at $184.74, on pace for its largest one-day percentage drop on record, according to Dow Jones Market Data. Merck shares slipped less than 1 percent. Moderna and Merck declined to comment.

The full dataset is now public ahead of the conference presentation scheduled for Saturday. What analysts argue about next is not whether the vaccine worked in this trial. It is how well it can work in the harder cancers both companies are testing it against.