A nurse takes a swab sample from a resident for COVID-19 test at a testing site in Jiangning District of Nanjing, capital of east China's Jiangsu Province, July 25, 2021. Nanjing, a mega-city with a population of more than 9.3 million, has launched a second round of all-inclusive nucleic acid testing and urged residents not to leave the city unless necessary. (Xinhua/Ji Chunpeng)