The Chinese city of Wuhan, where Covid-19 was first detected, will test its entire 11 million population after the virus was detected in the community for the first time in over a year.
It comes as several cities across China recorded local infections, with more than 400 cases combined in nearly two weeks, providing the Chinese government with one of its toughest challenges since the pandemic began.
Wuhan underwent one of the world's most stringent lockdowns as the virus rapidly spread at the start of 2020, however authorities managed to eliminate the virus with its last local case detected in May last year.
Yet the city was put on high-alert on Monday after the highly-infectious Delta variant was detected in the community as three case emerged.
"To ensure that everyone in the city is safe, city-wide nucleic acid testing will be quickly launched for all people to fully screen out positive results and asymptomatic infections," Wuhan official Li Qiang told a news briefing.
The news came as a shock to millions on Chinese social media site Weibo, many saying it is unfortunate for the city's residents to potentially face similar measures to last year.
"This hurts, it's all too familiar," one person wrote online.
Photos of packed supermarkets showed residents rushing to get supplies over fears restrictions could be imposed over the city.
Many however said the city's experience last year will put them in good stead to fend off the Delta variant.
"Wuhan has sufficient experience in epidemic prevention, we can definitely defeat the virus!" one user said.
Delta variant spreading across China
The new cases in Wuhan, along with infections in the nearby cities of Jingzhou and Huanggang since Saturday, were linked to cases in the city of Huaian in Jiangsu province, said Li Yang, vice director of the Hubei province disease control centre.
Cases of the variant first emerged in July in the city of Nanjing where airport workers are believed to have contracted the virus after cleaning a plane from Russia.
As seen in other countries, including Australia, the Delta variant has proven unrelenting, and authorities in Jiangsu believe "a laxity of mind" and complacency has allowed the virus to spread.
China has successfully managed to eradicate the virus in the past 18 months whenever it has reappeared in the community with fast, stringent measures as well as widespread testing.
But a Nanjing official said on Monday that even after the first cases were reported there, some shops did not rigorously check customers' digital health credentials and some did not wear masks properly.
Tourist destination Zhangjiajie, famous for its striking landscape and association with blockbuster film Avatar, has seen a surge in cases, with the Global Times reporting there are "grave concerns" over the outbreak.
China's top respiratory disease specialist Zhong Nanshan is reportedly alarmed over positive cases attending a performance at the theatre which attracts thousands of visitors a day.
Zhangjiajie said on Tuesday that residents and tourists should not leave the city, effectively imposing a lockdown.
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