Moderna and Merck have announced that their personalised mRNA cancer vaccine succeeded in a Phase 3 trial for melanoma, potentially marking the first major breakthrough for both mRNA vaccine technology and individually tailored cancer treatments. The trial tested the vaccine, intismeran, combined with Merck's established immunotherapy drug Keytruda, against Keytruda alone in patients whose melanoma had been surgically removed. The companies say the combination significantly extended the time patients remained free of cancer recurrence and distant spread, though full data has not yet been released.

The Phase 3 trial enrolled 1,137 patients with stage IIB–IV melanoma, randomised 2-1 to receive either the combined treatment or Keytruda alone over roughly a year, in a placebo-controlled, double-blind design. Intismeran works by encoding up to 34 mutations unique to a patient's tumour into synthetic mRNA, prompting healthy cells to produce protein fragments that train the immune system to attack cancer cells. Earlier five-year Phase 2 data, presented 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; full Phase 3 results are due to be presented at an upcoming medical conference. Notably, the companies describe the treatment as "neoantigen therapy" rather than an mRNA vaccine, a framing that avoids terms opposed by US Health Secretary Robert F. Kennedy Jr, who has cut federal funding for mRNA vaccine research.