Dr. Wafaa El-Sadr, a professor of epidemiology and medicine at Columbia University’s Mailman School of Public Health and director of ICAP, told McClatchy News over the phone that the wide availability of self testing “has completely changed the picture overall.” “We don’t know the number of tests that are done and we don’t know how many are positive, how many are negative,” El-Sadr said. “So it’s a whole kind of black box that makes looking at case rates really very unreliable.”