Merck and Moderna's Melanoma Vaccine Cuts Relapse Risk 42%, but Moderna Shares Fall 25%

Merck and Moderna reported on October 22, 2026 that intismeran autogene plus Keytruda (pembrolizumab) reduced the risk of recurrence or death by 42% in melanoma patients in a Phase 3 trial, according to Merck. Moderna shares fell 25.2% to $183.88 on the news, which Reuters reported as the stock's steepest one-day percentage decline. The company's market value dropped by about $24 billion.

The trial, INTerpath-001, enrolled more than 1,100 patients, according to Reuters. Reuters put the reduction in risk of death or recurrence at more than 40% and the reduction in risk of melanoma spread at 38%. Merck's own release gives the 42% figure for recurrence or death. The companies first announced on August 19, 2026 that the trial had met its recurrence-free survival (RFS) and distant metastasis-free survival (DMFS) endpoints. An endpoint is the outcome a trial is designed to measure. RFS tracks how long a patient lives without the cancer returning. DMFS tracks time without spread to distant sites in the body.

The regimen is adjuvant, meaning it is given after surgical resection, or removal of the tumor, to patients at high risk of relapse. Merck first described the Phase 3 design in 2023 as a study of V940 (mRNA-4157) with Keytruda in resected high-risk Stage IIB-IV melanoma. A second Phase 3 trial, started in 2024, tests the adjuvant combination after neoadjuvant Keytruda and chemotherapy, which is given before surgery. Moderna and Merck have nine Phase 2 and Phase 3 trials of intismeran with Keytruda underway, per Merck's June 2026 release on five-year data.

The earlier numbers are easy to conflate with the Phase 3 readout, so they need separating. In June, five-year data from a prior mid-stage study showed the vaccine cut the risk of cancer return by half, as Reuters reported. Fierce Biotech reported that the primary analysis of that Phase 2 trial showed a 65% reduction in the risk of distant metastasis or death. That is a different study, with a different population and a smaller sample, and 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 states that intismeran is the first and only combination regimen to show a clinically meaningful improvement over pembrolizumab alone, a claim the company makes.

Merck and Moderna plan to present the Phase 3 results at the ESMO 2026 Presidential Symposium. That presentation is a future event, and the full dataset, including absolute event rates and survival curves, will be available to the market only then. The topline figures are relative risk reductions. A 42% relative reduction in recurrence or death says nothing on its own about the absolute difference in recurrence rates between the two arms. Oncologists, payers, and regulators will want that absolute difference before judging the size of the benefit.

The share reaction is the more interesting puzzle for investors. The topline was public on August 19, so the October release was the first detailed look at the magnitude of the effect, yet the stock fell by a quarter on the day. One reading is that the market had already priced in a positive readout and the detail disappointed relative to expectations, the classic sell-the-news pattern. Another is that investors were reassessing how much of Moderna's valuation rests on this single program, and the detail gave them more to reassess. The move could reflect both. Anyone weighing the stock should separate the clinical result, which is what the trial measured, from the expectations embedded in the price, which is a separate and harder question.

For patients, the immediate meaning is narrow. The data apply to adjuvant use after surgery in high-risk melanoma, and the trial results have not yet been reviewed in full. The broader question for the sector is whether a personalized mRNA vaccine given alongside a checkpoint inhibitor can separate itself from pembrolizumab alone in a way that regulators and payers accept. The ESMO presentation is where that case will be tested in detail.