(Bloomberg) -- The Chinese metropolis of Chengdu will lock down its 21 million residents starting Thursday night, forbidding anyone without a special exemption from leaving their homes.  

It’s the biggest city to shut since Shanghai’s bruising two-month crisis earlier this year, reflecting the widening disruption in China’s continued Covid Zero battle. The country’s vast Western region has hitherto been largely untouched by the coronavirus.

Mass testing will begin Thursday evening. Residents must stay home indefinitely unless they have an exceptional situation, such as a medical emergency, the city government said. Families can send one person out - once per day - for groceries. They can leave the city only after getting a negative test result within the past 24 hours. 

Covid has spread widely within China thanks to its increasing contagiousness, even as the number of infections dwindle in response to aggressive measures to eliminate all traces of it. The results underscore the need to act quickly to interrupt transmission, despite the increasingly heavy economic cost the measures are inflicting on the world’s second-largest economy. 

The lockdown comes as the city reported 157 cases for Wednesday. 

Read more: Covid Zero Oasis Chongqing Shows Why Xi Keeps Locking Down

While the approach is being challenged by more contagious subvariants, and exacting an increasingly heavy social and economic toll, Chinese President Xi Jinping has reaffirmed his commitment to Covid Zero. There’s additional pressure to swiftly tame flareups in the run up to the party congress later this year, when Xi is expected to secure a precedent-breaking third term as leader.

Total infections in Chengdu in the current outbreak have exceeded 700, according to numbers released by the local health commission. Residents who have special reasons or virus prevention jobs can take public transportation with a negative Covid test result from the previous 24 hours. 

Due to the extremely hot weather in the city, frequent contacts among people in indoor entertainment venues including water parks led to increased transmission of the virus, Yang Xiaoguang, head of Chengdu’s health commission told reporters during a press conference on Tuesday. 

Chengdu postponed school restart times on Wednesday, as did multiple cities across the country, citing complicated virus prevention efforts in the city. 

(Adds the total number of infections in the second to last paragraph)

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