A first-of-its-kind cancer vaccine is significantly reducing the chances that melanoma patients' tumors come back, according to results of a major study released Thursday by drugmakers Moderna and Merck.

The vaccine was custom-brewed for each patient to match their tumor.

Adding it to a standard post-surgery treatment reduced by 42% the risk of melanoma returning two years after surgery, according to the most advanced study to date of this new strategy for fighting cancer.