With swift shots to the arm, doctors safely and effectively prime our immune systems to fight off deadly infectious diseases. Now, with tightly crossed fingers, they plan to do the same for cancers.

In two early clinical trials involving 19 patients with skin cancer, personalized vaccines appeared safe and effective at spurring immune responses to attack and destroy tumors. The vaccines worked by coaching killer immune cells—T cells—to destroy tumors by seeking out uniquely mutated proteins on each patient’s one-of-a-kind cancer cells, while leaving healthy cells unharmed.

The results of the two trials, both published this week in Nature, follow years of basic research and animal studies on this strategy. Researchers are optimistic, but there are big hurdles ahead of these small trials, including bigger trials with more patients and controls. If those go well, researchers will likely have to figure out how to streamline creating vaccines for individual patients, which is currently tedious and expensive.