On Sept. 10, a new case of the Delta variant of COVID-19 appeared in China, in the southeast province of Fujian. By the end of the day on Sept. 13, a total of 102 cases had been identified. Most of them are in Xiamen, a city of 5 million people, and a transport hub for the region. Mass testing of the whole population has begun, along with a mobilization to trace and track each infection, leading to targeted quarantines and lockdowns.