Doctors say about 15 percent of people who get Covid-19 still have symptoms more than three months later. They call the phenomenon "long Covid" but, until now, there really hasn't been a clinical definition of what that means. Last week, researchers at Harvard University Medical School published a paper that outlines what qualifies as long Covid and what doesn't. Tanayott Thaweethai is the lead author of that paper, and he spoke with the CBC's Leigh Anne Power.