A novel personalized cancer vaccine has shown promising results in significantly reducing the risk of melanoma recurrence, according to data released Thursday by pharmaceutical companies Moderna and Merck. The vaccine, named intismeran, is tailored to each patient’s tumor based on its unique genetic mutations, and when added to standard post-surgical immunotherapy, it lowered the chance of melanoma returning by 42 percent within two years.

The findings stem from a large, international late-stage clinical trial involving more than 1,100 patients diagnosed with high-risk melanoma who had undergone surgical removal of their tumors. All participants received Merck’s immunotherapy drug Keytruda, with half also given the custom mRNA vaccine in a series of nine doses administered every three weeks, while the remainder received a placebo.

Two years after treatment, 82 percent of those in the vaccine group remained cancer-free, compared to 70 percent in the group receiving only the standard therapy. Researchers emphasize that the study represents the most advanced evaluation to date of using personalized mRNA vaccines to stimulate the immune system to recognize and attack tumor-specific neoantigens—proteins produced by mutations unique to each patient’s cancer cells.

“This is a big win for the field,” said Dr. Hussein Tawbi of MD Anderson Cancer Center in Houston, one of the study’s lead investigators. He noted that side effects reported were generally mild and consistent with those seen in mRNA COVID-19 vaccines, including fatigue and localized soreness.

Moderna and Merck had disclosed the trial’s success in August, but the detailed results were originally scheduled for presentation at the European Society of Medical Oncology meeting in Madrid later this week. Preliminary data was leaked Thursday, prompting the companies to release full findings ahead of the conference.

Experts not involved in the study regard the results as a significant advance. Dr. Michael Atkins of Georgetown University described it as “a big step forward for science and oncology,” while Karen Knudsen, CEO of the Parker Institute for Cancer Immunotherapy, called it “a new frontier” with potential applicability to other cancers such as lung, kidney, and pancreatic tumors.

The vaccine manufacturing process involves sequencing tumor DNA to identify mutations and then producing a bespoke mRNA sequence that trains the immune system to detect these markers. Moderna’s president, Stephen Hoge, explained that though vaccine production itself takes only a few days, the overall process—including tumor analysis—can take approximately six weeks.

One participant, Tony Hout of Cayuga, Indiana, who joined the study in 2024 after his melanoma was detected during surgery for a persistent ear lesion, remains cancer-free following treatment. He travels regularly to MD Anderson for follow-ups and expressed hope that the vaccine will keep him healthy for his family’s sake.

Regulatory discussions are underway regarding approval of intismeran, though the companies have yet to comment on the vaccine’s potential cost. Moderna’s Massachusetts facility is currently producing doses for multiple ongoing international trials involving thousands of patients.

This study marks an important milestone in the development of personalized therapeutic cancer vaccines, an approach distinct from traditional vaccines that prevent infection by targeting cancer-causing viruses. By harnessing mRNA technology originally developed for COVID-19 vaccines, researchers are advancing new strategies to reduce cancer recurrence and potentially improve long-term survival for patients with melanoma and other malignancies.