No new coronavirus variants were detected in China during the Lunar New Year holiday, when millions of people returned to their hometowns, a mass migration that potentially helped the virus spread in the world’s most populous country.

The virus circulating during the holiday, which began on Jan. 21, consists of older omicron strains known as BA.5.2 and BF.7, said Chen Cao, a researcher at the Chinese Center for Disease Control and Prevention, at a briefing in Beijing on Monday. There were 1,421 viral sequences submitted for analysis by various provinces across the country during the seven-day holiday, Chen said.

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Those strains have been driving the outbreak for weeks, the Chinese CDC said. Its surveillance published on Monday shows BF.7 accounted for 70% of cases and BA.5.2 made up 28% of infections between December and Jan. 23, based on the sequencing of 10,165 samples. While the testing also captured a sliver of the XBB and BQ.1 subvariants, which have emerged as dominant strains elsewhere in the world, no others were found in China during this period.

Public health officials worldwide have been worried about new variants emerging from China as a result of the virus’ blistering spread among its 1.4 billion people. Several countries reinstituted testing requirements for travelers coming from China after Beijing dismantled its COVID restrictions in one fell swoop in early December. The move triggered a cascade of transmission that reached 80% of the population in less than two months, according to Wu Zunyou, the chief epidemiologist at the China CDC.

The officials also said China’s outbreak has been waning since it peaked in late December, with no obvious rebound during the Lunar New Year. The country’s hospitals saw a record 128,000 new severely ill patients admitted on Jan. 5, with a steady decline to 36,000 on Jan. 23. COVID-related deaths in hospitals peaked at 4,273 on Jan. 4, and has since dropped nearly 80% to 896 on Jan. 23.

The China CDC said it has consolidated data on COVID infections, severe disease and deaths after local authorities stopped mandatory mass testing at the end of last year. It’s gathering information based on tests run at laboratories across the country and rapid antigen tests done in the home, as well as treatment and mortality reports from hospitals.

Bloomberg’s Claire Che contributed to this report.