Using mRNA tailored to each patient’s tumor, the vaccine may have staved off the return of one of the deadliest forms of cancer in half of those who received it.

Five years ago, a small group of cancer scientists meeting at a restaurant in a deconsecrated church hospital in Mainz, Germany, drew up an audacious plan: They would test their novel cancer vaccine against one of the most virulent forms of the disease, a cancer notorious for roaring back even in patients whose tumors had been removed.

The vaccine might not stop those relapses, some of the scientists figured. But patients were desperate. And the speed with which the disease, pancreatic cancer, often recurred could work to the scientists’ advantage: For better or worse, they would find out soon whether the vaccine helped.

On Wednesday, the scientists reported results that defied the long odds. The vaccine provoked an immune response in half of the patients treated, and those people showed no relapse of their cancer during the course of the study, a finding that outside experts described as extremely promising.

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