The epidemic in Tonghua City was detected early but developed rapidly. The outbreak was directly related to Mr. L, and had a clear intergenerational transmission relationship.
Most of the individuals in the outbreak were retired elders from small and medium-sized cities. The joint epidemiological investigation found that these individuals lived a somewhat sedate lifestyle, had a similar socioeconomic level, and were eager to participate in external activities. They took free buses to other locations, greatly expanding their geographic range for activities. Because the course participants were elderly, the outbreak had a high proportion of severe and critical cases. By the end of January, 47 severe or critical cases were reported, accounting for 15.2% of the total cases, including 1 death of an 87-year-old person on January 25, 2021.
Three major factors contributed to this outbreak: First, Mr. L had muscle pain and visited a pharmacy on January 10 to obtain medication. This indicated that Mr. L was likely at an early stage of disease when holding lectures on January 10 and January 11. Early stage illness is believed to be most infectious (2). Second, most of the participants in his classes were elderly, with a median age of 72 years. The classes lasted long and were held in a small, confined space. Third, personal protection was not utilized, and social distancing were not maintained. The classroom was crowded, with about 30 people in a 30 square meters space. Mr. L and many participants did not wear masks during class. The outbreak was relatively confined, and no new virus strains were identified through genetic sequencing. This investigation showed, therefore, that there was no evidence of increased virulence or transmission.
In winter, the life of retired people in the north can be sedate and routine. Participating in educational events is an attractive activity. Thus, imported viruses can spread easily and widely. We suggest that local governments strengthen infection control management and reduce unnecessary indoor large gathering activities during wintertime, especially in the north of China. Health education for elderly people should be strengthened to promote use of effective personal protection.
Acknowledgements: Feng Tan, Xudong Li, and Lance Rodewald from China CDC Weekly.
Feild teams on case and close contact tracing: Tonghua City CDC Team; Jilin Province CDC Team; Jilin City CDC Team; Jilin City, Changyi District CDC Team; Jilin City, Chuanying District CDC Team; Jilin City, Longtan District CDC Team; Jilin City, Fengman District CDC Team; Siping City CDC Team; Siping City, Lishu County CDC Team; Siping City, Shuangliao City CDC Team; Siping City, Yitong County CDC Team; Meihekou City CDC Team; Liaoyuan City CDC Team; Liaoyuan City, Dongfeng County CDC Team; Liaoyuan City, Dongliao County CDC Team; Baishan City CDC Team; Baishan City, Jiangyuan District CDC Team; Baishan City, Fusong County CDC Team; Baishan City, Jingyu County CDC Team; Baishan City, Linjiang County CDC Team; Changbai County CDC Team.