An experimental mRNA vaccine allowed people with high-risk melanoma to live longer, cancer-free, the pharmaceutical companies Moderna and Merck announced Wednesday.

For decades, cancer vaccines have been one of the most tantalizing ideas in cancer treatment — offering the hope of harnessing a person’s own immune system to attack tumors.

“It’s a big deal,” said Dr. Elias Sayour, a pediatric oncologist at the University of Florida, who was not involved in the trial. The results, he said, “could help usher a whole new wave of new therapeutic treatments for cancer.”

The mRNA technology that was instrumental in the rapid vaccine development in the Covid-19 pandemic added a new element of speed and flexibility to the quest for therapeutic cancer vaccines. Scientists can use the mRNA platform to create bespoke, individualized vaccines matched to the genetics of patients’ tumors.

The companies announced only the broad outline of the results of the large, closely watched trial and plan to present the details at an international medical meeting and to regulators.

In the late-stage trial of 1,137 people with high-risk melanoma who underwent surgery to have their cancer removed, those who received the personalized cancer vaccine, called intismeran, along with an immunotherapy drug called Keytruda were less likely to see their cancer recur or spread compared with those who received just Keytruda.

Scientists create intismeran by sequencing the patient’s tumor and selecting what are called “neoantigens,” which teach the immune system how to attack the cancer. They use the fast, flexible mRNA technology to create a vaccine with neoantigens selected for each patient — a process that takes about six weeks.

“It is certainly gratifying to see these exciting results — to come full circle, from our early proof-of-concept results to now,” Dr. Catherine Wu, a professor of medicine at Harvard Medical School and the Dana-Farber Cancer Institute and a leader in the field, wrote in an email. In 2017, Dr. Wu’s lab demonstrated the promise of customized vaccines that train a fighting force of immune cells to target the specific neoantigens on an individual patient’s tumor.

“That was the starting point of the field. These were small studies, less than 20 patients,” Dr. Wu said. She added that the next challenge would be to find a way to extend the approach to other types of cancer that may be harder to treat than melanoma, a deadly skin cancer with an estimated 112,000 new cases per year in the United States.

The study is the most advanced of a number of trials testing the approach against a variety of cancers, including lung, bladder, kidney and pancreatic cancer. While small trials have shown the promise of the approach, the Moderna and Merck trial was seen as a bellwether in demonstrating the technology — and a first step toward transforming cancer care with treatments that are individualized to a person’s tumor and that may ultimately be able to stay ahead of it as it changes.

“Folks like myself in the community have been waiting for this,” Dr. Sayour said, adding, “We believed in this, we expected this, but I think there’s tremendous capacity for enhancing these approaches.”