Moderna and Merck said their personalized mRNA vaccine, mRNA-4157, delayed melanoma recurrence and spread in a Phase 3 trial, the first randomized late-stage readout for a cancer vaccine built from a patient's own tumor DNA. The mechanism is not a generic shot. A biopsy is sequenced, software predicts which mutated proteins (neoantigens) sit on the surface of that specific patient's tumor cells, and a custom mRNA strand coding for up to 34 of them is synthesized and injected alongside Merck's checkpoint inhibitor Keytruda, training the immune system to recognize a tumor that does not exist in any other patient. That is why it cannot be stockpiled: each dose is a one-off manufacturing run keyed to one person's biopsy.

The trial sits at Phase 3, randomized against Keytruda alone in resected high-risk melanoma, which is a materially stronger claim than the Phase 2b data that first moved Moderna's stock in 2023. Full trial data, including the hazard ratio for recurrence and the size of the survival benefit, is due when the companies present results at a medical meeting later this year. Until that readout, "delayed recurrence" is a topline label, not a number regulators can act on.