A personalized mRNA cancer vaccine combined with the immunotherapy Keytruda succeeded in a late-stage trial for the deadly skin cancer melanoma, its makers said Wednesday, potentially opening up a new way of treating the disease.

Moderna shares more than doubled Wednesday after it announced initial results from a study of an experimental cancer treatment that would be the first of its kind if approved by regulators.

The vaccine, tailor-made to the genetic sequence of a patient’s cancer and paired with drugmaker Merck’s existing treatment Keytruda after the cancer was surgically removed, has the potential to be a ...