A large clinical trial combining Moderna 's mRNA cancer vaccine with Merck 's immunotherapy reduced the risk of melanoma death or recurrence by 42%, the companies announced on Thursday, potentially opening a new era in personalized oncology.

Moderna shares closed more than 17% lower on Thursday after the trial findings leaked on an oncology website, despite having nearly quadrupled through Wednesday's close since August when the trial's success was first announced without full data. Merck stock also fell about 1.8%.

The Phase 3 trial, which tracked more than 1,100 patients, also revealed that the combination treatment reduced the risk of the cancer spreading by 38%.

The findings mark the first data demonstrating that pairing a custom vaccine based on a patient's unique tumor mutations with a drug that boosts the immune system can successfully halt the disease.…