Moderna Shares Plunge 25% After Leak of Melanoma Vaccine Data Ahead of ESMO

Shares of Cambridge-based Moderna fell about 25 percent on Thursday, Oct. 22, 2026, after detailed results from its personalized melanoma vaccine trial with Merck surfaced online two days before their scheduled presentation in Madrid, showing a 42 percent reduction in the risk of cancer returning or death, a result doctors praised and investors judged merely good.

A leak, then an early release

The numbers were supposed to stay under embargo until a late-breaking session at the European Society for Medical Oncology congress on Saturday, Oct. 24. Instead, according to Yahoo Finance, they appeared on the medical news site Oncology Central around 12:30 p.m. Eastern on Thursday and were taken down shortly afterward. Screenshots circulated on social media, BioPharma Dive reported, and ESMO responded by releasing the closely guarded study details ahead of the meeting rather than letting a partial version stand.

Moderna declined to comment on the leaked report, and neither company immediately confirmed the figures, according to Yahoo Finance. The full presentation in Madrid remains scheduled for Saturday.

What the trial found

The Phase 3 study, known as INTerpath-001, enrolled 1,137 patients with high-risk stage IIB to IV melanoma whose tumors had been surgically removed, Yahoo Finance reported. Patients received either Merck’s immunotherapy Keytruda alone or Keytruda plus intismeran autogene, the individualized mRNA vaccine Moderna and Merck have developed together. The vaccine is built for each patient by sequencing the tumor and encoding up to 34 of its unique mutations, or neoantigens, to train the immune system to attack any remaining cancer cells.

After a median follow-up of 24.6 months, the combination cut the relative risk of recurrence or death by 42 percent compared with Keytruda alone, a hazard ratio of roughly 0.58, according to BioPharma Dive and Yahoo Finance. At two years, 82.2 percent of patients on the combination were alive and cancer-free, versus 70 percent on Keytruda alone. The risk of the cancer spreading to distant organs fell by 38 percent.

Side effects were broadly similar between the groups, though vaccine recipients reported more fever and injection-site pain. About 7 percent of patients on the combination stopped treatment because of an adverse event, compared with 5 percent in the control arm, BioPharma Dive reported. STAT News, reporting from Madrid, said oncologists described the result as an important advance because patients lived significantly longer without their cancer returning.

Why a positive result sank the stock

The answer is expectations. Moderna and Merck announced on Aug. 19 that the trial had met its primary and key secondary goals without releasing figures, and Moderna’s stock rose 93 percent in a single day on that news, according to 24/7 Wall St. By Wednesday’s close of $245.81 the shares had more than quintupled in 2026. Wall Street had been looking for a risk reduction of 40 to 50 percent, BioPharma Dive reported, and the earlier Phase 2b study had shown 44 percent at a comparable point and 49 percent after five years. A 42 percent result landed at the low end of the range, and slightly below the mid-stage data.

Analysts split along predictable lines. Myles Minter of William Blair described a “sell-the-news reaction,” according to BioPharma Dive. Tyler Van Buren of TD Cowen called the data a “highly compelling efficacy profile.” Trung Huynh of RBC was the most skeptical, writing that the result sat at the low end of expectations and that commercial success is now less certain, even though he still considers approval likely. Yahoo Finance noted that UBS had said a month earlier that the market wanted a hazard ratio of roughly 0.65 or better, a bar the trial cleared comfortably.

The stock traded near $185 on Thursday afternoon after being briefly halted for volatility, Yahoo Finance reported, and 24/7 Wall St. put the shares at $184.87 in afternoon trading, still up roughly 526 percent for the year. Merck, a far larger company for which the vaccine is a smaller piece of the story, fell as much as 3.9 percent intraday and closed down about 1 percent. The two outlets differ on Merck’s move only because one cites the intraday low and the other the close.

What the doctors said

Trial investigators framed the data differently from traders. Hussein Tawbi of MD Anderson Cancer Center said that until now no drug had improved on Keytruda’s results in this setting, and that the trial shows it can be done, according to BioPharma Dive. Janice Mehnert of NYU Langone, another investigator, said many patients would accept the added side effects in exchange for a longer time without relapse. Roughly a third or more of late-stage melanoma patients still relapse or die within five years even on Keytruda, which is the gap the vaccine is meant to close.

What comes next for Moderna

Neither company has published a regulatory filing timeline, and Yahoo Finance said investors will watch submission timing as the next marker. Intismeran is also in Phase 2 and 3 trials in non-small cell lung cancer, bladder cancer and kidney cancer; BioPharma Dive reported that interim kidney results could arrive in late 2026 or the first half of 2027. The broader question for the field, as STAT put it, is how widely the personalized mRNA approach can be applied beyond melanoma. BioNTech’s rival personalized vaccine failed a mid-stage colorectal cancer trial in August, a reminder that the technology is not yet a sure thing in every tumor type.

The Boston Made Take

For Kendall Square, Thursday was a strange kind of good news. Moderna, the company that defined the pandemic era in Cambridge and then watched demand for its COVID vaccine fade, now has a Phase 3 result in a second disease area that oncologists are calling an advance. That is the outcome the region’s biotech ecosystem has been waiting for. The 25 percent drop says less about the science than about a stock that had risen more than fivefold on a headline and needed the actual numbers to be exceptional rather than solid. The leak made the reset sharper and uglier than it had to be, and ESMO’s decision to publish early rather than let screenshots drive the narrative was the right call. The questions that matter for the brand now are commercial ones: how fast the companies file, how a vaccine manufactured one patient at a time gets priced and produced at scale, and whether the lung and kidney readouts land on the same side of 40 percent.

What it means for you

• Patients and families: the data covers the adjuvant setting, meaning after surgery for high-risk melanoma. It is not yet an approved product, and treating oncologists will be watching the Saturday presentation in Madrid for the full dataset.
• Investors: the stock remains up more than 500 percent this year even after Thursday’s drop, which means volatility around the regulatory filing and the next trial readouts is likely to stay high.
• Cambridge and Greater Boston employers: a credible oncology franchise at Moderna supports hiring in manufacturing, bioinformatics and clinical operations after a stretch of cuts across the local biotech sector.
• Brand watchers: Moderna’s reputation is being rebuilt around personalized medicine rather than COVID; how the company communicates the leak and the data this week will shape that story.