Moderna Falls 17.6% After Leaked Melanoma Vaccine Data Shows 42% Risk Reduction

MRNA shares fell as much as 25% on Thursday, October 22, an intraday record, after partial data from the Phase 3 trial of its personalized melanoma vaccine began circulating from unverified sources on X ahead of its scheduled presentation. The stock ended the session down about 17.6% at $202.63, while partner MRK dropped 3.9%.

The leaked figure showed that intismeran autogene, given with Merck's Keytruda, reduced the risk of melanoma recurrence or death by 42% compared with Keytruda alone. Merck subsequently confirmed the key data points, adding that the combination cut the risk of the cancer spreading to other parts of the body, or death, by 38%. The stock fell even though the full data, released later on Thursday, appeared positive.

The disappointment is about expectations rather than activity. Investors compared the 42% figure with an earlier Phase 2b study that showed a 49% reduction in recurrence or death and a 59% reduction in distant spread, although that study involved a different group of patients over a different period, so the two are not a direct comparison. Ahead of the readout, coverage framed the trial as one on which a lot was riding for Moderna's cancer ambitions, which raised the bar the data had to clear.

The next marker is the full presentation at the European Society for Medical Oncology Congress in Madrid on Saturday, October 24. Details on durability of benefit, subgroups and safety will shape whether the market treats the result as a commercial-grade win for Moderna's oncology franchise or as a smaller step than the rally implied. Moderna, Merck and ESMO did not immediately comment on the leak.