Science’s COVID-19 reporting is supported by the Heising-Simons Foundation. After the first surge of COVID-19 cases in spring 2020, a new worry emerged: Some people didn’t get better. For those with so-called Long COVID, lingering symptoms ranged from brain fog and intense fatigue to shortness of breath and loss of smell and taste. So far, there’s little clarity about what causes or how to treat this constellation of symptoms. Some surveys suggest between 10% and 30% of people infected with the pandemic coronavirus may struggle to recover, but these data are preliminary. Emilia Liana Falcone, an infectious disease specialist at the Montreal Clinical Research Institute, and Michael Sneller, an infectious disease specialist at the National Institute of Allergy and Infectious Diseases (NIAID), are each leading a large Long COVID clinical trial. They are recruiting volunteers who’ve had COVID-19—some with ongoing symptoms and some without—along with a control group of people who never caught the virus. Volunteers come in regularly for medical tests, and scientists probe their blood for immune abnormalities. The goal: a biological explanation of chronic symptoms after COVID-19. The pair spoke with