Moderna Shares Drop More Than 25% on Apparent Early Release of Cancer Vaccine Data
Moderna shares plunged more than 25% after an apparent early release of cancer vaccine data, and Merck shares fell more than 3% on the same news, according to Investing.com, which reported the move on October 22, 2026.
The product at the center of the sell-off is intismeran autogene, also called mRNA-4157 and V940. It is a personalized mRNA cancer vaccine that Moderna and Merck are developing together. It is studied in combination with Merck's Keytruda (pembrolizumab), an immunotherapy drug that helps the immune system go after tumors.
The most recent formal disclosure on the program came on August 19, 2026. Moderna and Merck announced that the Phase 3 INTerpath-001 trial of intismeran plus Keytruda met its recurrence-free survival (RFS) and distant metastasis-free survival (DMFS) endpoints in melanoma, according to a joint announcement from Moderna. A Phase 3 trial is a large, late-stage study designed to test whether a treatment works. An endpoint is the outcome a trial is built to measure. RFS tracks how long a patient goes without the cancer returning, and DMFS tracks how long it goes before spreading to distant parts of the body. Reuters reported that the results came from an interim analysis, meaning the data were examined before the trial was fully complete. Moderna shares spiked as much as 160% intraday that day and Merck shares jumped 12%. The next day, Fierce Biotech reported that Moderna had closed up 177%.
The August readout sat on a longer development path. Moderna and Merck reported Phase 2b KEYNOTE-94 results in December 2022, when the combination met its primary efficacy endpoint. In December 2023 the companies initiated the pivotal Phase 3 INTerpath-001 trial, which asks whether mRNA-4157 plus pembrolizumab is more effective than pembrolizumab alone. In January 2026, Merck reported five-year data showing sustained improvement in recurrence-free survival. Earlier five-year data from a mid-stage trial, presented in June, showed the vaccine cut the risk of cancer returning by half, per Reuters.
Scope matters here. Merck describes the INTerpath program as nine Phase 2 and Phase 3 trials spanning multiple tumor types and stages of disease. Keytruda itself is used against 20 different kinds of cancerous tumors and 45 types and stages of cancer, according to Reuters.
The broader context here is that a melanoma win sits inside a much larger commercial and clinical frame. That is why the stock reactions in August were so large.
In my view, the October move is a different kind of event. A scheduled topline announcement comes with a defined data set and a company statement. An apparent early release comes with neither. Traders have to weigh two things at once: what the material says, and how much credit to give material that has not been formally presented. A 25% single-session decline in Moderna suggests the market treated that uncertainty as serious, even before the content of any formal update is known. Merck's smaller decline points the same way, though the two moves are not directly comparable, since the companies' exposure to the program differs.
The sensible reading for anyone following the stock is narrow. The August Phase 3 result rested on an interim analysis. Neither that fact nor the October episode tells a reader what the next formal data will show, or when it will arrive. Large swings in a single company tied to one clinical program are the expected outcome of binary-type news, meaning results that can land strongly in either direction, and the price action reflects that.
