But for months, it seemed clear that the contagion had been all but eradicated across much of China, with daily life largely returning to pre-pandemic normality.

Chinese officials have kept a lid on the pandemic by deploying a zero-tolerance playbook that would be considered extreme in the West: sealing off entire cities and enforcing mandatory testing on tens of millions of people.

The current wave started July 10, officials said, when nine airport workers tested positive after having cleaned an Air China flight that arrived in the eastern city of Nanjing from Moscow.

Officials have once again revved up the zero-tolerance machine, banning anyone from leaving the city of Zhangjiajie, canceling flights and trains across the country and shutting tourist sites. Nevertheless, infections have spread to 15 of China’s 31 provinces, state media reported Friday.