Media in China say researchers at Peking University estimate that as many as 900 million people in the country have been infected with the coronavirus. That's more than 60 percent of the population.
The researchers say they came to a cumulative figure as of January 11 based on data such as the number of online searches related to COVID-19 symptoms.
They say infections spread rapidly after the government eased coronavirus restrictions on December 7, and peaked 13 days later in many places.
Authorities have not updated official infection figures since January 8.
Many people in China head to their hometowns for the Lunar New Year holidays beginning January 21.
Concerns are growing about the spread of infections in rural areas and the emergence of a new strain.