A Cancer Vaccine Trial Looked Strong, Yet Moderna's Stock Fell by a Quarter

Merck and Moderna said on October 22, 2026 that a combination treatment cut the risk of melanoma (a serious skin cancer) coming back or patients dying by 42%. The combination pairs Moderna's personalized vaccine, called intismeran autogene, with Merck's drug Keytruda (pembrolizumab). The figure comes from Merck.

Moderna's shares fell 25.2% to $183.88 that day. Reuters reported it as the stock's steepest one-day percentage drop on record. The company's market value fell by about $24 billion.

The trial is called INTerpath-001. It enrolled more than 1,100 patients, according to Reuters. Reuters put the reduction in the risk of death or recurrence at more than 40%, and the reduction in the risk of the melanoma spreading at 38%. Merck's own release gives 42% for recurrence or death.

The companies first said on August 19, 2026 that the trial had met two main goals. The first is recurrence-free survival, or RFS, which measures how long a patient lives without the cancer returning. The second is distant metastasis-free survival, or DMFS, which tracks how long passes without the cancer spreading to far-off parts of the body.

The treatment is adjuvant. That means it is given after surgery removes the tumor, to patients at high risk of the cancer coming back. Merck first described the study design in 2023. It tests V940 (mRNA-4157) with Keytruda in resected, high-risk Stage IIB to IV melanoma. A "Phase 3" trial is the large, late-stage test that comes before regulators decide whether a treatment can be approved. A second Phase 3 trial, started in 2024, tests the combination after Keytruda and chemotherapy. Merck and Moderna have nine Phase 2 and Phase 3 trials of intismeran with Keytruda underway, per Merck's June 2026 release on five-year data.

Several earlier numbers are easy to mix up with the Phase 3 result. In June, five-year data from an earlier, mid-stage study showed the vaccine cut the risk of cancer returning by half, as Reuters reported. Fierce Biotech reported a 65% reduction in the risk of distant spread or death from that Phase 2 trial's main analysis. That was a different study, with different patients and a smaller group, so its figure should not be read as the Phase 3 result. CNBC described the August readout as coming from the first-ever late-stage trial of the personalized vaccine. Moderna also says intismeran is the first and only combination to show a clinically meaningful improvement over pembrolizumab alone. That is the company's claim.

Merck and Moderna plan to present the full Phase 3 results at the ESMO 2026 Presidential Symposium. That event has not happened yet. Until then, the public will not see the complete data, including the actual rates of cancer return and the survival curves.

The 42% is a relative figure, and that matters. Think of a store sign that says "prices cut 42%" without showing the old price. It tells you the direction and rough size of the change, but not the actual amounts. A 42% drop in risk does not tell you how many more patients stayed cancer-free in each group. Doctors, insurers, and regulators will want that absolute difference before they judge how much the drug helps.

The share reaction is the harder puzzle for investors. The main results were public on August 19, so the October release was the first detailed look at how large the effect was. Yet the stock still fell by a quarter that day. One explanation is that investors had already priced in a good result, and the details fell short of what they expected. Traders call this "sell the news." Another explanation is that investors were rethinking how much of Moderna's value depends on this one program, and the new details gave them more to rethink. Both explanations could be true at once. In my view, the useful split is between the clinical result, which is what the trial measured, and the expectations built into the share price, which is a separate and much harder question.

For patients, the immediate meaning is narrow. The data apply to use after surgery in high-risk melanoma. The full trial results have not yet been reviewed.

The broader context here is a bigger question for the whole field. Can a personalized mRNA vaccine given alongside a checkpoint inhibitor (a drug that helps the immune system attack cancer) show enough of an edge over Keytruda alone to satisfy regulators and insurers? The ESMO presentation is where that case will be tested in detail.