Four districts in Chongqing began testing all residents on Sunday to curb its latest COVID-19 outbreak. The southwestern metropolis reported 18 confirmed locally transmitted cases and 16 asymptomatic carriers as of Sunday.

The virus is the Omicron BA.5.1.3 variant, local authorities said at a news conference on Sunday. It is currently the most prevalent strain in the world. The variant was also detected in Hainan province recently, the first time this variant has been detected on the Chinese mainland.

"Compared with other variants, it is more contagious, spreads faster and is harder to discover," said Luo Fei, chief physician at the Chongqing Center for Disease Control and Prevention. "So this time the infected area is very broad and the infected people had a rather complex activity trajectory."

At present, all infected people have been hospitalized and are in stable condition, local health officials said at the conference.

Chongqing is a densely populated city with 31 million people. Local residents are urged to report to their communities if they have overlapping trips with infected people.