Chinese authorities reported 1,337 locally transmitted cases of COVID-19 across dozens of mainland cities Monday as the fast-spreading variant commonly known as “stealth omicron” fuels China’s biggest outbreak in two years.

The vast majority of the new cases were in far northeastern Jilin province with 895. Shenzhen reported 75 new cases as residents began the first of three rounds of mass testing. Officials on Sunday locked down the city, which has 17.5 million people and is a major tech and finance hub that neighbours Hong Kong.

The surge on the Chinese mainland is infecting people in cities ranging from Shenzhen to Qingdao on the coast, to Xingtai in the north and the numbers have crept steadily higher since early March. While the numbers are small relative to numbers reported in Europe or in the U.S., or even the city of Hong Kong, which had reported 32,000 cases Sunday, they are the highest since the first big outbreak of COVID-19 in the central city of Wuhan in early 2020.