Connor Johnson was eager to return to his job as a licensed electrician after recovering from COVID-19 last spring. But his enthusiasm was short-lived when, only three weeks after going back to work, he was diagnosed with acute myeloid leukemia (AML), a type of blood and bone marrow cancer.

“I knew something was wrong, because my smartwatch kept alerting me that my heart rate was elevated,” he recalls. “I’d rest a while, then I...