Shenyang, Liaoning’s capital, also reported one confirmed case on Sunday.

Wu Zunyou, chief epidemiologist at the Chinese Centre for Disease Control and Prevention, said on Saturday night that Yingkou was likely the source of the outbreak even though Luan was the first to report one on Thursday.

Last year, Yingkou reported a number of local transmissions linked to imported cases, and Wu did not rule out the possibility that this was the situation again.

“New outbreaks in China have [in the past] been imported from overseas, by people or goods,” he said.

Li Yan, deputy director of Yingkou’s CDC, said the people diagnosed in the city had not been vaccinated and authorities were still trying to identify “patient zero”.

04:12

What do we know so far about the Covid-19 variants?

What do we know so far about the Covid-19 variants?

Checkpoints have been set up at 10 highway toll stations between Yingkou and Dalian, requiring drivers and passengers to show a negative result of a test taken within the previous 72 hours to leave Yingkou and enter Dalian.

Mass testing has been carried out in the affected cities.

Bayuquan district in Yingkou has ordered the closure of kindergartens and schools, and suspended in-person private training programmes.

Zhao Wei, a professor of public health at Southern Medical University, said the pandemic was still a serious concern and China could not take prevention and control lightly.

“Danger is always there. Foreign exchanges are still relatively close, and national borders can not be so tightly closed,” he said.

02:06

After ramping up vaccination drive with incentives, China administers 200 million Covid-19 shots

After ramping up vaccination drive with incentives, China administers 200 million Covid-19 shots

The latest outbreak has prompted a rush on vaccines, with Hefei authorities saying residents were turning out in record numbers to get the shots.

Hefei mother Fan Feng said she had been reluctant to get inoculated because there had been no cases in the city, but the new infections had changed that.

“It’s [suddenly] hard to get vaccinated ... We will probably have to wait several days. The queue was already long at 7.30 in the morning on Saturday and remained long despite the heavy rain during the day,” Fan said.

Teresa Xu, a 32-year-old English teacher based in Hefei, said she was also now thinking about getting the shots.

She said she was in Guangzhou in southern Guangdong province on a business trip and had been contacted by police inquiring about her plans and asking her to get a test.

“If I don’t get vaccinated, I will almost certainly be asked to show my nucleic acid test report in other places, which is troublesome,” Xu said.

Do we really need coronavirus vaccine booster shots?

China has set a target of having 40 per cent of the population vaccinated by July but it was not clear how far it has to go to reach that goal.

“Many people have not been willing to get vaccinated, which is a problem, but we should learn a lesson from this round of cases that the virus is not far from us. If so, maybe a bad thing will turn out to be a good thing,” Zhao with the Southern Medical University said.

Zhong Nanshan, a prominent respiratory specialist who was vaccinated on Friday, called on people to have the jabs as soon as possible.

Meanwhile, officials in Yuan district in Luan are under investigation for failing to prevent the outbreak. Doctors who received patients with a fever in violation of coronavirus control regulations would also be suspended and held legally responsible, according to a municipal statement.

Zhao said the situation had been controlled well in the country as a whole but the public and authorities had relaxed.

“Which is understandable, but now due to the new outbreak, we need to again stress the need for a strict policy at fever clinics,” he said.

“The cases have spread from Liaoning to Anhui, probably also to the surrounding provinces, and the local transmission has been happening for a while, so we should closely monitor any new cases.”