About 80,000 tourists are stranded on the Chinese island of Hainan, which local authorities call “the Hawaii of China,” after a surge in Covid-19 cases triggered a lockdown. All flights leaving the city of Sanya have been canceled since Saturday. Photo: Guo Cheng/Zuma PressSINGAPORE—China’s southern technology hub of Shenzhen is shutting down the city center over the weekend, as more parts of the country tighten Covid-19 controls to battle a new wave of infections just weeks ahead of a high-profile Communist Party congress. Shenzhen officials have suspended subway and bus service in the area and are telling most residents to...