An asymptomatic novel coronavirus case infected at least 141 other people in a superspreading event during a recent COVID-19 epidemic in Northeast China, highlighting the need to restrict indoor gatherings in northern areas in winter, according to a report published by the Chinese Center for Disease Control and Prevention.

The patient, an unidentified 44-year-old product promoter, directly infected at least 81 people in two days last month by giving lectures in a concealed classroom and other activities in Tonghua, Jilin province, the report said.

Meanwhile, epidemiological investigations revealed that at least 60 others were infected by close contacts of the lecturer during activities such as dining, shopping and playing mahjong, said the report, which was published online on Feb 10 at China CDC Weekly.

The lecturer tried to promote sales of a flax oil product, and many elderly people were lured to attend by the promise of free eggs, Xinhua News Agency reported.

He gave three lectures on Jan 10 and 11 in a Tonghua classroom while not wearing a mask, with each lecture lasting about 2.5 hours. A total of 97 people, most of them retirees, attended the lectures in a classroom whose doors and windows were closed, the China CDC report said. The lecturer went to a pharmacy at noon on Jan 10 due to muscle pain and other symptoms, indicating he was likely at an early stage of the disease then, it said.

By Jan 31, 74 people who attended the lectures had tested positive for the novel coronavirus. At two of the lectures, attended by 71 people, 90 percent of those at each event tested positive.

Some major factors contributed to the outbreak, including a lack of personal protection measures, such as the lecturer and many participants failing to wear masks and a lack of social distancing in the crowded and concealed environment, the report said.

Despite the rapid spread of the virus during the outbreak, investigations identified no new virus strains and no evidence of increased virulence or transmission, it said.

Yin Dapeng, an author of the report and a China CDC epidemiologist, said the superspreading event exposed the potential risks of major indoor gatherings in winter, and intensified epidemic prevention and control measures were needed to minimize such risks.

"We suggest that local governments strengthen infection control management and reduce unnecessary large indoor activities during winter, especially in the north," he said. "Health education for elderly people should be strengthened to promote the use of effective personal protection."