Moderna Sinks 17% After Late-Stage Melanoma Vaccine Data Leaks Prematurely

Moderna (MRNA) shares plunged more than 17% Thursday, the biggest decline in the S&P 500, after late-stage trial data for its melanoma vaccine leaked on OncologyCentral before the company's planned disclosure. Partner Merck (MRK) fell nearly 2%. The companies subsequently issued a press release stating that Intismeran, combined with Merck's Keytruda, "significantly reduced the risk of recurrence or death by 42%" in the Phase 3 trial. Jefferies analysts called the data "encouraging" in a note to clients Thursday. UBS said the results raised its confidence in the program while noting the selloff may reflect overly high investor expectations. Moderna shares had doubled in August after early positive results from the same study. The full data was scheduled for presentation Saturday at the 2026 European Society for Medical Oncology congress. Despite Thursday's drop, Moderna remains up nearly 600% year to date; Merck has gained 40% over the same period.