A medical worker collects a throat swab from a resident at a nucleic acid testing site in Shunyi district in Beijing, on Dec 26, 2020. (Photo: Xinhua)

The recent cluster of local confirmed coronavirus cases in Beijing's Shunyi district was caused by an imported asymptomatic case from Indonesia, based on epidemiological investigation and genomic sequencing, a senior official said at a news conference on Wednesday.

Pang Xinghuo, deputy head of the Beijing Center for Disease Control and Prevention, said the authority has locked down the infection source and chains of virus transmission.

"The investigation of the virus samples from the cases and environment showed that the virus is not genetically related to local cases in past months, nor from the UK or South Africa," she said. "It's highly related to a virus that was discovered from cases in Southeast Asia in November this year."

Pang said the source of the virus transmission is a 28-year-old male who lived in Indonesia and arrived in East China's Fujian province on Nov 26.

He was a close contact of an imported confirmed case on the flight when he traveled back.

He completed the 14-day centralized observation in Fujian with a negative nucleic acid test result. Then, he came back to Beijing on Dec 10 and stayed in Shunyi district.

After the new cases occurred in Shunyi starting on Dec 23, the district conducted mass scale testing in the district. He got tested on Dec 26 with the result still showing negative, but environmental samples from his living space and workplace showed positive.

He then got another test on Monday. The result was positive and he was confirmed as an asymptomatic case.

"This round of cluster is single transmission. The source infected another case, who is his roommate in Beijing, and this roommate infected others while shopping," Pang said.

According to the authority, since Shunyi district reported its first case on Dec 23, it has reported 16 cases in total by Tuesday. They all were infected on the transmission line from the source.

Shunyi district has collected about 1.33 million test samples since Dec 23, among which, 1.30 million have so far been confirmed as negative.

Zhi Xianwei, executive deputy head of the district, said the district will start testing for the second time people in key areas where the cases were reported.

"By 3 pm Wednesday, 24,600 samples have been collected for the second time," he said.

He said the district plans to give no less than three rounds of testing on people in the key areas.

In addition, another asymptomatic case reported in Xicheng district on Dec 24 is not related to the source in Shunyi and that investigation is still going on.

Beijing reported one locally-transmitted case on Tuesday, a 4-year-old male living in Chaoyang district. He is receiving medical treatment at Ditan Hospital and his two close contacts are under centralized medical observation.

The online car-hailing app Huaxiaozhu announced on Wednesday that it will suspended its service for one week in Beijing to organize drivers on the platform can get nucleic acid tests. A confirmed case reported on Monday was a driver for the online car-hailing app.