The Omicron variant of the coronavirus started circulating in the northern Chinese port city of Tianjin during the three-day New Year holiday, health authorities said on Wednesday as the city of 14 million launched a second round of mass testing. Officials said contact tracing indicated that children who went to day care facilities during the holiday contracted the virus and spread it at school when they returned on January 4. “The virus was passed to their schools, to their schoolmates and to their respective families, resulting in a spread to different communities,” Tianjin Centre of Disease Control and Prevention deputy director Zhang Ying said. Zhang said this explained why the onset of symptoms in most Covid-19 patients in the ongoing outbreak was from January 8 to 10, when there was a plateau in cases. “This means that the testing efforts and Covid-19 controls in Tianjin were in place early and rapidly. The upcoming development of the outbreak can be controlled as long as our measures are spot on,” she said. “We launched the second round of citywide testing [to eliminate] the community transmission risk by picking up infections that had yet to spread.” The first round of mass testing started on Sunday morning and identified 77 cases among 12.5 million samples. Tianjin is under enormous pressure to ensure its outbreak does not spread to neighbouring Beijing, which will host the Winter Olympics in about three weeks. The port city announced on Wednesday that it would postpone its annual “ two sessions ” political meetings scheduled for this Sunday and Monday. All primary, secondary and tertiary school classes have been suspended. Tianjin reported a total of 137 Covid-19 infections on Wednesday afternoon, including 17 asymptomatic cases. Jinnan district accounted for 130 of the cases. The authorities said the cases showed a rather clear transmission chain – two index cases, five cases detected in mass testing and the rest close contacts of patients and staff at close-off areas. There are 11 family clusters.