Shanghai reported the first local case of the Omicron subvariant known as BA.5 on Sunday with an infection related to previous imported cases.

Zhao Dandan, deputy director of the Shanghai Health Commission, said a COVID-19 infection registered in Pudong district on Friday was found to be BA.5.2.1.

Experts said that BA.5 strain is a highly contagious strain feared for more rapid transmission speed and immunity-evading ability.

As the recent outbreak related to a karaoke bar in Xuhui district has led to more than 200 infections since July 3, the city will implement mass nucleic acid tests in at least nine districts, including downtown Huangpu, Jing'an and Xuhui, twice from Tuesday to Thursday to stop potential hidden spread, Zhao said.

"The city is still at a critical juncture in bringing the epidemic under control," said Yuan Zheng'an, a member of the expert group with the city's epidemic prevention and control leadership group.

Also on Sunday, Shanghai reported one locally transmitted COVID-19 asymptomatic infection outside of quarantine and locked down communities. The infection is a 61-year-old man residing in Xuhui's Huajing township.

As Shanghai's temperature climbed to 40 C on Sunday, the city released its first red alert for heat this year. The Shanghai Health Promotion Center reminded residents to replace masks when necessary, as it is easy for them to get wet due to sweat on scorching days.

It also urged residents to keep abiding epidemic prevention and control protocols, including social distancing and scanning venue health QR codes, while avoiding heat exposure especially around noon.