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GUANGZHOU, Aug. 20 (Xinhua) -- More than 617,700 nucleic acid test samples have been collected in the city of Shenzhen, south China's Guangdong Province, as part of the latest batch of COVID-19 testing since a new case was detected earlier this month, authorities said on Thursday.

The testing started on Aug. 14 after a staff member at a local store of Alibaba's fresh-food chain Freshippo tested positive for the virus. The city launched a special nucleic acid screening campaign and conducted tests on relevant personnel and units in its cold-chain market.

As of Thursday morning, with 590,200 samples tested so far, three came back positive, according to the municipal health commission.

The three asymptomatic cases have been put under isolated medical observation in a local hospital.

Shenzhen reported one new asymptomatic case of COVID-19 on Wednesday. The city had reported 465 confirmed cases as of Wednesday, of which three are currently receiving treatment.

Also on Thursday, the Nantang Township of Lufeng City in Guangdong was upgraded to a COVID-19 medium-risk area as the township recently reported one confirmed case and four asymptomatic cases.