A novel mRNA cancer vaccine helps cut the chances of melanoma tumor recurrence in major study

Lauran Neergaard · The Associated Press

· Posted: Oct 23, 2026 11:10 AM EDT | Last Updated: 19 minutes ago

A first-of-its-kind cancer vaccine is significantly reducing the chances that melanoma patients' tumors come back, according to results of a major study released Thursday by drugmakers Moderna and Merck.

The vaccine was custom-brewed for each patient to match their tumor. Adding it to a standard post-surgery treatment reduced by 42 per cent the risk of melanoma returning two years after surgery, according to the most advanced study to date of this new strategy for fighting cancer.

"The field has been looking for a win, and I think this one is a big one," said Dr. Hussein Tawbi of MD Anderson Cancer Center in Houston, one of the researchers in the international study.

The vaccine, called intismeran, is made with the same Nobel Prize-winning mRNA technology that was behind COVID-19 vaccines. Experimental personalized cancer vaccines made with mRNA and some other approaches are also being tested against other types of cancer, including lung, kidney and pancreatic.

Experts not involved with the new study say its success bolsters the field.

"This is a new frontier," said Karen Knudsen, CEO of the Parker Institute, which helps fund cancer immunotherapy research.

"It's a big step forward for science, for oncology, because this vaccine will apply to many different cancers," said Dr. Michael Atkins, deputy director of Georgetown University's Lombardi Comprehensive Cancer Center.

When combined with an immunotherapy treatment, the vaccine reduced recurrence by 42 per cent, the study states. Experts say this new vaccine advances the field of cancer prevention. (Dado Ruvic/Illustration/Reuters)

Moderna and Merck had announced in August that the late-stage trial of intismeran was successful.

The detailed results were set to be released this Saturday at a meeting of the European Society of Medical Oncology in Madrid, but after some of the data was leaked publicly late Thursday, the companies provided a full release ahead of the weekend's scientific presentation.

Inside the personalized vaccine

Most people think of vaccines as preventing an infection, like measles or whooping cough. There are already two cancer-preventing vaccines, the HPV and hepatitis B shots that block infection from those tumor-causing viruses. And research is underway to prevent some types of inherited cancer such as Lynch syndrome-caused colorectal cancer.

But a personalized vaccine designed to block surgically-removed cancer from returning is very different. For the intismeran study, scientists hunted gene mutations unique to each patient's tumor — mutations that create proteins that shouldn't be in the body unless those cancer cells start to grow again. The custom vaccine — nine doses given three weeks apart — teaches the patient's immune system to watch out for and attack those so-called neoantigens.

WATCH | Moderna and Merck announce results in August:

How did it work? The study included more than 1,100 people with high-risk melanoma that was surgically removed. Everyone was given the standard follow-up therapy: Merck's immunotherapy named Keytruda. The participants were randomly assigned to also receive custom-brewed vaccine doses or a placebo.

Two years later, 82 per cent of the people in the vaccine group were still living without a recurrence of their cancer compared to 70 per cent who had gotten standard Keytruda alone.

"This is highly statistically significant. It is not by chance," Tawbi, the MD Anderson Cancer Center melanoma expert, told Reuters of the results.

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The trial is continuing to track participants' longer-term outcomes.

In 2024, Tony Hout joined the vaccine trial at MD Anderson after seeking care for a spot on his ear that had been bleeding intermittently and wouldn't fully heal. Doctors recognized melanoma and during surgery discovered it had started to spread.

"I want to make sure I'm doing everything I can to be here for my family," said Hout, 62, who has since moved to Cayuga, Ind., and travels back to Houston every few months for a study check-up. Scans this week showed he's still cancer-free.

Tony Hout, left, talks with Dr. Hussein Tawbi at the University of Texas MD Anderson Cancer Center in Houston, on Monday, in this video still. (Lekan Oyekanmi/The Associated Press)

The vaccine's side effects were mainly the sore arm, fatigue and other reactions typical of mRNA coronavirus vaccination, MD Anderson's Tawbi said.

Moderna and Merck are talking with regulatory authorities about next steps for seeking approval of the vaccine.

Shares drop after leak

Customized medicines generally are very expensive. The drugmakers aren't discussing what such a cancer vaccine might cost.

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But Moderna president Stephen Hoge said the company's special cancer vaccine manufacturing facility in Massachusetts is providing doses for thousands of people enrolled in multiple studies underway internationally.

Hoge said it takes a few days to brew the vaccine, but the whole process, starting with analyzing the tumor genetics to come up with what is essentially a recipe for each patient, can take roughly six weeks, sometimes more. Meanwhile, the melanoma patients had already started their standard Keytruda, a different kind of immunotherapy.

WATCH | Moderna stocks climb after initial announcement:

Moderna shares have soared since the companies in August announced the trial's success without the actual data, nearly quadrupling through Wednesday's close.

After the study’s full results were leaked Thursday on an oncology website, Moderna shares dropped 18 per cent, but remained more than 200 per cent higher than they were before investors were first told in August of the initial results. Merck shares closed down about 1.8 per cent.

Several Wall Street analysts said before the meeting that investors were looking for the combination of the vaccine and Keytruda to cut the risk of recurrence more, Reuters reported.