Scientists are on the cusp of fulfilling a decades-long quest: to use vaccines to teach the immune system to spot and destroy cancer cells. News that an mRNA cancer vaccine developed by Merck & Co. and Moderna Inc. kept melanoma from returning in a large, late-stage trial marks a critical next chapter for a technology beleaguered by post-pandemic political backlash and years of false starts before then. The concept of cancer vaccines has been kicked around for over a century, and in the past 20 years, we’ve seen countless hopeful headlines about promising early-stage treatments. None of it panned out. Coaxing the body into fighting its own battles turned out to be a lot harder than it sounds. mRNA is finally moving the field beyond the “hope” and “promise” phase. Merck & Moderna’s Phase 3 study combining the personalized mRNA-based vaccine intismeran with the immunotherapy Keytruda is offering the evidence needed to prove that cancer vaccines really can work. There’s still plenty to do to show their potential extends beyond melanoma and into other kinds of cancer. But there