(Adds Moderna's comments on potential to seek accelerated approval in paragraphs 2-3, Phase 3 confirmatory study in paragraph 8)

By Julie Steenhuysen

CHICAGO, June 5 (Reuters) - Adding an experimental mRNA-based vaccine from Moderna Inc and Merck & Co reduced the risk that the most deadly skin cancer would spread by 65% over treatment with an immunotherapy alone in a mid-stage trial, the companies reported on Monday.

With this and earlier data, Moderna is considering seeking faster approval from regulators for the treatment, the company told investors after having presented the results at the American Society of Clinical Oncology meeting in Chicago.

"Some of the residual uncertainty seems to be going away on that potential (option)," said Moderna President Stephen Hoge.

The data followed earlier promising data from the trial showing the customized mRNA vaccine given in combination with Merck's Keytruda cut the risk of death or recurrence of melanoma by 44% compared with Keytruda alone.

The findings add to a growing body of evidence suggesting that mRNA technology, which rose to prominence during the COVID-19 pandemic, can be used to assemble personalized vaccines that train the immune system to attack the specific type of cancer cells in a patient's tumors.

Scientists have been chasing the dream of vaccines to treat cancer for decades with few successes. Experts say mRNA vaccines, which can be produced in as little as eight weeks, paired with drugs that rev up the immune system may lead to a new generation of cancer therapies.

The hope is for "a completely new treatment paradigm in cancer that will be better tolerated and unique to individual patients' tumors," said Dr. Jane Healy, an executive overseeing in early cancer treatment development at Merck.

Moderna said during its investor call that it was starting a Phase 3 confirmatory study, which it hoped to open in the third quarter of this year.

The Merck/Moderna collaboration is one of several combining powerful drugs that unleash the immune system to target cancers with mRNA vaccine technology. Pfizer's COVID vaccine partner BioNTech SE and Gritstone Bio Inc are taking similar approaches using mRNA technology.

The vaccines all target neoantigens, new mutations that are only present on tumors. Aiming at these unique proteins allows the immune system to kill cancer cells while leaving healthy tissue unscathed.

The trick is determining which of many mutations is driving the cancer.

To accomplish this, tumors are removed and their genetic makeup is mapped using next-generation DNA sequencing. Companies use artificial intelligence to predict which mutations will be the most effective targets. These are used to build an individualized vaccine targeting only mutations in the patient's tumor.