An experimental personalized skin cancer vaccine from Merck and Moderna significantly reduced the risk that melanoma would return or spread in a late stage clinical trial. The companies said the result marks the first positive Phase 3 trial of an mRNA personalized cancer therapy.

Known as INTerpath 001, the study evaluated 1,137 patients around the world after surgery to remove high risk melanoma. Participants had Stage IIB through Stage IV melanoma.

Patients were randomly assigned to receive either Keytruda with the personalized vaccine or Keytruda with a placebo. Those receiving the vaccine combination experienced significantly better recurrence free survival, meaning their cancer was less likely to return than among patients receiving Keytruda alone.

The combination also significantly improved distant metastasis free survival. That measure reflects how long patients remained alive without their cancer spreading to distant parts of the body.

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"These phase 3 findings represent a pivotal moment for the field of cancer research," Stéphane Bancel, CEO of Moderna, said in the press release. "For many years, the idea of creating an mRNA treatment designed specifically for an individual patient's cancer was aspirational. We are now helping turn that vision into a reality."

Unlike traditional vaccines that protect against external viruses, this treatment is intended for use after surgery. The custom mRNA vaccine, known as intismeran autogene, is designed to train the body to hunt remaining cancer cells.

After surgery to remove a tumor, doctors sample cancerous tissue and healthy cells from the patient. Advanced algorithms then analyze the genetic sequence of the tumor and select up to 34 tumor specific neoantigens to encode in the personalized treatment.

Dr. Murad Alam, chief of dermatology at Northwestern Medicine in Chicago, said melanomas can differ from one patient to another. That variation is central to the individualized approach being tested.

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"They're slightly different in every patient, which is why traditional chemotherapies often stop working or don't work in a particular patient," Alam told Fox News Digital. "This is almost individualized immunotherapy, where you're finding specific issues in a particular patient."

The approach is designed to guide the immune system toward targets specific to each tumor. Alam said the vaccine is created to recognize the signs of an individual patient's cancer and attack cancer cells without striking normal cells.

"It doesn't hit normal cells, and it hits the cancer cells very effectively because it's not just a generic vaccine. It's one that specifically knows the signs of your cancer and can find it," he said.

According to Alam, the vaccine is intended to extend the lives of people who already have a serious case of melanoma. It is not designed to prevent melanoma in people without the disease, including those with a family history, or to treat those with a very small melanoma.

"This is designed for people where the melanoma has grown to the point where it's threatening not just the local area of the skin, but their whole lives," Alam said.

Artificial intelligence also plays a role in determining which antigens should be included in each vaccine. Alam said the technology makes it feasible to perform the difficult work for individual patients within a reasonable amount of time and with reasonable resources.

Following the successful Phase 3 outcome, Merck and Moderna announced plans to engage with regulatory authorities worldwide, including the U.S. Food and Drug Administration, about potential regulatory filings. Plans are also underway to test the vaccine technology against lung, bladder and kidney cancers.

Despite the positive results, the findings remain early and did not show how long patients stayed cancer free. The Phase 3 announcement also did not establish whether the treatment ultimately helps patients live longer.

The recurrence free survival results were based on investigator assessments rather than an independent central review. In addition, the study involved only people with specific stages of skin melanoma who had undergone surgery and received no prior drug treatments, meaning the results may not apply to other cancer types or stages.

Merck said the side effects were similar to those observed in earlier studies, with no new safety concerns identified. Alam described the vaccine as having a "tremendous benefit [and] good side effect profile," while noting mild fatigue, chills and pain at the injection site.