When the patient came back 10 days later, coughing repeatedly and complaining of headache, Davey Smith feared the worst. Smith had prescribed the patient Pfizer’s new antiviral pill, Paxlovid, on the previous visit, after a COVID-19 test came back positive. A resurgence of symptoms probably meant one thing, especially after Smith tested the patient and got another positive. “I was pretty sure it was resistance,” said Smith, an infectious disease physician at the University of California, San Diego. “I’m a virologist, I combat resistance all the time.”