“The current outbreak is very serious and complex and poses a major risk of further spread,” Gai Dongping, vice-mayor of Jilin, told reporters in an online briefing on Wednesday.

“In order to prevent and stem the spread of the epidemic, the Jilin epidemic prevention and control group has decided to implement control measures in the urban area of Jilin,” she said.

That included closing all residential compounds and rural villages in the area, the Jilin city government said in a statement.

Anyone who wants to leave Jilin – the second biggest city in the province of the same name – must provide a negative report for a nucleic acid test done in the 48 hours before departure.

Social gatherings have also been banned and indoor public venues – such as theatres, internet cafes, mahjong parlours and public bathhouses – have been closed until further notice, the statement said.

The city’s education bureau meanwhile announced that all schools would be closed with immediate effect, and any group activities – such as training, tutoring, competitions or exams – have been banned. Some middle school and all high school students in the city had returned to the classroom last month after a lengthy shutdown, and more were due to go back soon. Now they will all be learning from home again.

Outdoor morning markets where locals shop for groceries would also be closed from Thursday to prevent people from gathering, the city’s urban management bureau said in a statement.

China has reported new cluster outbreaks in Heilongjiang, Jilin and Hubei provinces in recent weeks since it declared that local transmission of Covid-19 was “on the whole” under control.

He Qinghua, an inspector with the Bureau of Disease Prevention and Control at the National Health Commission, said the outbreaks exposed weaknesses in hospital infection and community controls, as well as personal protection measures.

“The country’s epidemic prevention and control work has changed from emergency response to a constant reality. This does not mean that our prevention and control measures can be relaxed or we can take a break. It’s not acceptable,” He said at a separate briefing in Beijing. “Epidemic prevention and control is still the top priority and a prerequisite to advance towards comprehensive restoration of social order.”

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How the woman contracted the disease is still a mystery since there had been no local cases reported for 73 days before she fell ill, and she had not travelled out of the province or been in contact with anyone returning from overseas.

Gai said three national-level epidemic prevention teams had arrived in Jilin city to work with experts from the province to investigate the outbreak.

“The nucleic acid samples of the 15 confirmed cases in the early stage were all genetically sequenced, and the mobile P3 laboratory test vehicle supported by the state will be put into use as soon as possible,” Gai said, referring to a testing facility for handling dangerous pathogens. “The national expert group is giving guidance on carrying out contact tracing and a fourth group is on its way.”

She said medical experts were working to identify the source of the outbreak and the chain of transmission as soon as possible.

The new coronavirus was first reported in the central Chinese city of Wuhan late last year, and has since infected more than 4.2 million people and killed over 291,000 worldwide.