Pharmaceutical partners Moderna and Merck announced on Wednesday that their novel mRNA-based vaccine—individually tailored to target a patient’s unique cancer mutations—was effective in a late-stage clinical trial of patients with melanoma, one of the deadliest forms of skin cancer.

The announcement was scant on details, but if the success holds, it stands to realize the high aspirations for both mRNA vaccines and individualized cancer-targeting medicines in cancer therapy, providing a first success for both treatment types.

The Phase 3 trial included 1,137 patients who had stage IIB–IV melanoma that had been surgically removed prior to their treatment in the trial. Patients were randomized two-to-one to receive either a combination treatment of the tailored mRNA vaccine (intismeran or mRNA-4157) with Merck’s established monoclonal antibody cancer treatment Keytruda, or they received treatment with Keytruda alone. Both patient groups were treated for about a year. The trial was placebo-controlled and double-blind, meaning neither the doctors nor the patients knew which treatment patients were randomly assigned to receive.

Based on a planned interim analysis, Moderna and Merck report that the combination therapy with the mRNA vaccine extended “recurrence-free survival” (RFS), which is the amount of time a patient has without the cancer returning. The combination therapy also extended “distant metastasis-free survival” (DMFS), which is the amount of time a patient has without the cancer returning and occurring in a different part of the body than where it began.

The companies say the extensions of RFS and DMFS reported were statistically significant and clinically meaningful, but they did not elaborate. They said the data will be presented at an upcoming international medical conference. They did note, however, that data from a five-year Phase 2 trial, which was presented at a cancer researcher conference in June, showed a 49 percent reduction in the risk of recurrence or death and a 59 percent reduction in the risk of distant metastasis or death.

The mRNA vaccine intismeran works using a synthetic mRNA that holds the unique genetic coding for up to 34 mutations in a patient’s own cancer cells. The production process involves comparing the genetic sequences of a patient’s cancer cells to those of healthy cells and picking out any cancer-specific mutations that could help the immune system differentiate cancer cells from healthy ones.