A personalised cancer vaccine is showing early signs it could keep deadly skin cancer from coming back for years.

New clinical trial results from Moderna, presented Monday at the American Society of Clinical Oncology’s annual meeting and published in the Journal of Clinical Oncology, found the mRNA vaccine halved the risk of melanoma returning after five years.

Melanoma is the deadliest form of skin cancer, and in about half of patients, the disease will come back within the first five years of treatment.

“The treatments we have are not perfect. People relapse,” said Dr Janice Mehnert, the director of the melanoma and cutaneous medical oncology program at NYU Langone Health in New York and the senior trial investigator.

In the trial, 50 patients received standard treatment of surgery followed by the immunotherapy pembrolizumab, also known as Keytruda. Another 107 patients also received a personalised vaccine tailored to their specific tumour.

All of the people in the trial had at least Stage 3 melanoma, meaning the cancer had spread to nearby lymph nodes or skin and had a high risk of returning even after surgery.

Five years later, nearly 70 percent of people in the vaccine group were cancer-free, compared to 49 percent of people in the standard treatment group. Adding the vaccine also cut a person’s risk of the cancer metastasising by nearly 60 percent.

The vaccine is designed to train the immune system to identify and kill lingering cancer cells and later any new cells that emerge.

Each vaccine is made using genetic material from the patient’s tumour, specifically DNA mutations in the cancer cells that cause unique proteins to form on the cells’ surfaces. These proteins, called neoantigens, are the targets the vaccine trains immune cells called T-cells to attack.

In the trial, each vaccine contained the information needed for the immune system to identify 34 of the top neoantigens that scientists believed would be the best targets.

Once the vaccine was ready, usually about four to six weeks after surgery, patients received up to nine doses spaced about three weeks apart. The doses aligned with their immunotherapy treatments.

Jeff Coller, a professor of RNA biology and therapeutics at Johns Hopkins University in Baltimore who was not involved with the research, said the results show that “the vaccine is doing exactly what we hoped it would do.”

“It’s training the immune system to recognize the tumor signature long after the tumor is gone,” he said.

The melanoma vaccines were in development before the Covid pandemic, but the research builds on advances made in mRNA technology over the last six years.

The trial was funded by Moderna and a subsidiary of Merck, which makes Keytruda.

Mehnert said results from a larger phase 3 trial involving 1,000 patients and expanded to Europe will provide the real proof of whether personalised mRNA vaccines added to standard immunotherapy significantly reduce the risk of melanoma returning.

All of the patients in the larger trial have finished treatment, and Mehnert and her team are compiling the results.

Dr Ravi Amaravadi, a professor of medicine at the University of Pennsylvania Perelman School of Medicine who specialises in melanoma patient care and was not involved with the trial, said the five-year data are still important because most cancers that are going to return do so in the first five years.

If there is no recurrence within that time, “we usually stop or slow down scanning,” Amaravadi said.

Dr Shailender Bhatia, director of the melanoma team at Fred Hutch Cancer Center in Seattle, who was not involved in the study, said one advantage the vaccine appears to have over other therapies is low toxicity.

“Generally what we have seen is that if we layer more drugs with immunotherapy, it causes more toxicity but not more benefit. So that is where the results of this trial are promising,” Bhatia said.

People in the trial reported side effects similar to mRNA Covid vaccines, including chills and headaches, that lasted only a couple of days.

Mehnert and her team found that people whose cancers did not return also had the most robust immune response after vaccination, suggesting it is the vaccine that boosted recurrence-free survival over five years.

In the future, Mehnert said her team would like to develop the personalised vaccines three to four weeks after surgery so patients can receive the therapy earlier. In the study, patients started receiving their vaccines during their third or fourth cycle of pembrolizumab.

But first, researchers need to see the results of the large-scale trial, Mehnert said.

If that trial yields similar results, it “will be paradigm-shifting,” said Bhatia.

“We have tried to use vaccines in cancer therapy for decades, but the efficacy has not been clinically relevant in phase 3 trials to date. If this is successful, this will open up a new field that will be relevant not just to melanoma, but many other cancers,” he said.

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