Moderna Loses $24 Billion in Market Value After Melanoma Vaccine Data Leaks on Social Media Two Days Early

Moderna shares plunged as much as 25% on Thursday after partial data from its closely watched Phase 3 melanoma vaccine trial, developed jointly with Merck, leaked on social media ahead of a planned presentation at the European Society for Medical Oncology conference in Madrid on Saturday. The leaked figures, first circulated by unverified accounts on X, showed a smaller risk reduction than the earlier study that had sent Moderna stock surging in August, triggering an immediate and violent sell-off. Trading in the shares was briefly halted due to volatility.

The full data, released by the companies later Thursday after the embargo was broken, appeared clinically meaningful and positive relative to Wall Street expectations, according to analysts. Even so, the stock finished the session down roughly 17% to 20%, depending on the source, with market cap losses totaling approximately $24 billion. Shares of Merck also fell on the news, while BioNTech dipped in sympathy. Pfizer edged modestly higher. Analysts described the reaction as a classic "sell-the-news" retreat, compounded by the fact that Moderna stock had skyrocketed after the company's initial August announcement of the vaccine program.

The broader market also fell sharply on Thursday, with the Nasdaq closing down 2.64% at 25,999.35, its biggest single-day percentage drop since June 5, losing more than $1.1 trillion in market value. The S&P 500 fell 1.24% to 7,661.33, while the Dow Jones Industrial Average ended nearly flat, down just 0.01% at 52,853.21. A separate wave of concern over AI spending weighed on technology stocks broadly, with Meta sliding roughly 5% and dragging other names lower.