Tianjin's disease control and prevention center has established a mature communication mechanism with its counterpart in Beijing to exchange information in time to ensure a smooth and successful 2022 Beijing Winter Olympic Games next month, a senior official said.

To contain the spread, particularly to the Chinese capital, which is 130 kilometers away, various departments have introduced prevention measures, said Zhang Ying, deputy director of the Tianjin Center of Disease Control and Prevention.

"Departments have introduced strict control and management measures in the Xinzhuang and Xianshuigu townships in Jinnan district after outbreaks were reported," she told media on Monday.

"Now the measures have been tightened further to prevent and control of the spread of the coronavirus there."

Meanwhile, railway departments are allowing all passengers who have purchased railway tickets to leave Tianjin for other mainland cities to get refunds starting Tuesday. People are encouraged to stay in Tianjin temporarily.

Zhang said Tianjin started a citywide testing campaign on Sunday to identify possible patients and asymptomatic carriers.

Tianjin had reported 31 confirmed cases and 10 asymptomatic carriers as of 6 pm on Monday, with some having been confirmed as the Omicron strain, Zhang said.

She said children do not appear to be more easily infected with Omicron than other strains, despite the fact that many of the cases are local students.

"Testing shows the proportion of children among infected people is decreasing, and the proportion of young adults, middle-aged people and elderly in the community is gradually increasing," she said. "Therefore, it does not appear that children are more susceptible to the Omicron variant. It should be said that the whole population is susceptible to the Omicron variant."

The Omicron strain is not only stealthy but also spreads very fast, with some foreign data showing the intergenerational days of transmission can be as short as 2 to 3 days, Zhang said. The Omicron strain might have existed and spread unobserved in Tianjin for some time, she added.

Government departments remain busy investigating the source of Omicron, whether it was directly imported to Tianjin from abroad or whether the inbound virus first went elsewhere on the Chinese mainland before spreading to the city, she said.