A personalized mRNA cancer vaccine, added to an existing treatment, slowed the return of melanoma and its spread to other parts of the body in a late-stage clinical trial, the drugmakers Merck and Moderna announced Wednesday, results that could herald a new, powerful approach in oncology.

So-called “neoantigen” vaccines have long been seen as having potential as cancer treatments, but this is the first randomized Phase 3 clinical trial aimed at definitively proving their benefit. In this case the personalized vaccine, intismeran, was combined with Merck’s Keytruda in adjuvant melanoma, meaning patients’ disease had been surgically removed.

If the results hold up — the drugmakers did not immediately release detailed data — they could deliver new hope and potentially longer lives for patients. They could also be a further testament to the power of mRNA as a platform to develop both traditional vaccines and therapeutics.