Authorities in Wuhan on August 3 said they would test its entire 11 million population for Covid-19 after the central Chinese city – where the coronavirus was initially detected – reported its first local infections in more than a year.

China is battling its largest coronavirus outbreak in months, confining the residents of entire cities to their homes, cutting transport links and rolling out mass testing as the fast-spreading Delta variant challenges its zero-Covid strategy and homegrown vaccines.

Beijing had previously announced its success in crushing the virus, allowing the economy to rebound and normal life to return while swathes of the globe have struggled with the pandemic that has killed more than four million people worldwide.

But the latest outbreak is threatening China’s success with more than 400 domestic cases reported since mid-July when a cluster among airport cleaners in Nanjing, Jiangsu province, sparked infections in over 20 cities across more than a dozen provinces.

Senior city official Li Tao told a press conference on August 3 that Wuhan is “swiftly launching comprehensive nucleic acid testing of all residents”.

Authorities announced on August 2 that seven locally transmitted infections had been found among migrant workers in the city, breaking a year-long streak without reporting domestic cases.

And the holiday destination of Zhangjiajie in central China’s Hunan province abruptly announced on August 3 that no one would be allowed to exit the city, after closing tourist attractions and encouraging visitors to leave last week.

China reported 61 domestic cases on August 3.