Experts estimate that Patient Zero, or the first patient to become ill in the coronavirus outbreak in North China's Hebei province, would have been infected before Dec 15, based on a study of gene sequencing and epidemiological survey.

The ongoing COVID-19 outbreak in Hebei province is spreading and hasn't reached its peak yet, and tracing work is ongoing, Shi Jian, head of the Hebei Provincial Center for Disease Control and Prevention's emergency management office, said on Sunday afternoon at the province's press conference on epidemic control.

Genetic sequencing of the virus found in confirmed cases in Shijiazhuang and Xingtai cities also showed the coronavirus strains belong to the European strain, Shi said. Also, it's not related to the recent outbreaks in some domestic cities, after comparison to the genes in the national data base.

This means it's highly possible that the infection was triggered by the virus outside the country, he said.

The genetic analysis also showed the virus did not have the variants discovered in the United Kingdom or South Africa, he added.

Hebei reported 40 new local confirmed cases and six asymptomatic carriers on Sunday as of 10 am, adding its local confirmed cases to 223 and asymptomatic carriers to 161, said Xu Jianpei, vice-governor of Hebei province.

Among the new confirmed cases, 26 were previously diagnosed with asymptomatic results, Xu said.