A district in central Beijing has gone into “wartime mode” after discovering a cluster of coronavirus cases around the biggest meat and vegetable market in the city, raising the prospect of a second wave of infections in the sensitive capital, the seat of the Chinese Communist Party. The discovery of dozens of infections, both symptomatic and asymptomatic, underscores the perniciousness of the virus and its propensity to spread despite tight social controls.

“We would like to warn everyone not to drop their guard even for a second in epidemic prevention control: we must be prepared for a prolonged fight with the virus,” Xu Hejian, a spokesman for the Beijing municipal government, said at a news conference Saturday. “We have to stay alert to the risks of imported cases and to the fact that epidemic control in our city is complicated and serious, and will be here for a long time,” he said.

Then on Friday, it announced that two quality control workers at the state-owned China Meat Food Research Center who had visited Xinfadi and five other markets in the city to check on standards had also tested positive.

This prompted a frenzy of testing. By the end of Friday, Beijing authorities had swabbed 1,940 workers in major supermarkets and other food markets in the capital, and collected 5,424 environmental samples.