Merck and Moderna¡¯s success with a personalized cancer vaccine designed to keep melanoma at bay opens a new avenue of treatment that doctors hope will work against many types of tumors.

Unlike chemotherapy, which kills both healthy and cancerous cells, or immunotherapy, which revs up the immune system, personalized cancer vaccines train the immune system to specifically target mutations found only on a person¡¯s tumor.

Merck and Moderna said on Wednesday their vaccine helped prevent the return and spread of disease in a large trial involving more than 1,000 melanoma patients. They had localized tumors that were removed surgically, but had a high risk of recurrence. Moderna expects that thousands of melanoma patients could benefit within the first few years of approval.