npr:

A spate of mysterious second-time infections is calling into question the accuracy of COVID-19 diagnostic tools even as China prepares to lift quarantine measures to allow residents to leave the epicenter of its outbreak next month. It’s also raising concerns of a possible second wave of cases.

From March 18-22, the Chinese city of Wuhan reported no new cases of the virus through domestic transmission — that is, infection passed on from one person to another. The achievement was seen as a turning point in efforts to contain the virus, which has infected more than 80,000 people in China. Wuhan was particularly hard-hit, with more than half of all confirmed cases in the country.

But some Wuhan residents who had tested positive earlier and then recovered from the disease are testing positive for the virus a second time. Based on data from several quarantine facilities in the city, which house patients for further observation after their discharge from hospitals, about 5%-10% of patients pronounced “recovered” have tested positive again.