On September 17, Mayra Arana made the phone call she says saved her life. Arana had been vaccinated against Covid-19, but she developed a breakthrough infection. She feared the virus might kill her, since her immune system is weak after years of treatment for leukemia. Arana's family physician in California told her there wasn't much she could do besides stay home and rest. At home, following her doctor's advice, Arana felt sicker by the hour. Her husband placed a pulse oximeter on her fingertip, and it showed her blood oxygen levels were dipping dangerously low. "I'm Catholic, so I kept...