The recent COVID-19 outbreak in Qingdao, Shandong province, was unlikely to have originated at the community level as no new positive result was found following citywide nucleic acid testing, officials said at a news conference on Thursday afternoon.

By 3:30 pm on Thursday, the city had collected and tested more than 10.4 million samples and had released more than 8.8 million results. There were no additional positive results beyond those already under quarantine, said Sui Ruwen, the city's vice-mayor and chief of the public security bureau.

The testing effort, which aims to cover the total population of 11 million residents, is expected to be finished on Friday, he said.

On Thursday morning, the city reported one new confirmed COVID-19 case, its 13th since Sunday. The case is an independent virus infection of a 40-year-old port worker whose test was positive but who had showed no symptoms three weeks ago.

"Generally, the incubation period of the virus ranges from one to 14 days, but there are also reported cases whose incubation period lasted four weeks," said Sun Yunbo, director of a medical experts team organized to treat the city's novel coronavirus, adding that the dock worker is an individual case.

In addition to the 13th confirmed case, the previous 12 in the recent epidemic, which started over the weekend, were closely linked to the city's pulmonary hospital. Some top health experts said the risk of infection spreading inside communities is unlikely.

The reasons for the recent outbreak should be analyzed by top health experts in the country, said Jiang Fachun, deputy director of the city's disease control and prevention center.

Wu Zunyou, chief epidemiologist at the Chinese Center for Disease Control and Prevention, said during an interview with Xinhua News Agency on Wednesday afternoon that efforts in the coming days should be focused on the close contacts of cases in the pulmonary hospital, especially if there are close contacts that have not been found out and isolated.

He suggested that the re-emergence of the virus in Qingdao is not due to the transition from autumn to winter, neither is it related to the eight-day National Day Holiday despite the large numbers of people who traveled during that time.

"Priorities for prevention should be on wards in hospitals, especially those receiving overseas confirmed cases, and the use of major medical equipment should also be isolated," said Wu.

"Wearing face masks, keeping social distances and washing hands frequently should become a daily routine," he added.

In response to the recent outbreak in Qingdao, Sui Zhenhua, director of the Qingdao health commission, was suspended from his post and is being investigated, and Deng Kai, dean of the pulmonary hospital, was removed, according to the statements from the city government on Thursday.