A cluster of COVID-19 infections indicating person-to-person transmission among casual contacts from social gatherings: An outbreak case-contact investigation

Shen, Ye; Xu, Wenjie; Li, Changwei; Handel, Andreas; Martinez, Leonardo; Ling, Feng; Ebell, Mark; Fu, Xiaofei; Pan, Jinren; Ren, Jiangping; Gu, Weiling; Chen, Enfu

SARS-CoV-2, the pathogen causing COVID-19, efficiently spreads from person to person in close contact settings. Transmission among casual contacts in settings such as during social gatherings is not well understood. We report several transmission events to both close and casual contacts from a cluster of seven COVID-19 cases occurring from mid-January to early February. A total of 539 social and family contacts of the index patient’s, including members of a two-day wedding and a family party, were contacted and screened through epidemiologic surveys. The clinical progression of all cases is described. We estimate the secondary attack rate among close contacts to be 29%(2/7) and for the casual contacts to be 0.6%(3/473). The incubation period of our case cluster was 4–12 days (median, 7 days). Transmission efficiency among close contacts was higher than among casual contacts; however, transmission from second-generation cases may help spread the virus during the incubation period.