Objective To analyze epidemic cluster of 2019 novel conronavirus diseases (COVID-2019) in Ningbo city of Zhejiang province and to provide evidence for effective containment of the epidemic.
Methods We collected the data on all COVID-2019 cases reported in Ningbo city from January 21, 2020 - the time of first case confirmation, till March 10, 2020. Descriptive statistics was conducted and incubation period and intergenerational interval of the reported cases were estimated.
Results During the period in the city, totally 22 COVID-2019 cluster epidemics were identified, involving 137 confirmed and 29 asymptomatic cases which were accounted for 88.77% of all (187) reported COVID-2019 cases. Of the cases involved in the epidemic clusters, 63.25% (105) were female and 59.64% (99) were aged over 50 years. Among all the involved cases, the proportion of reported infection-exposure history was 40.96% (68) for exposed to family members, 10.84% (18) to relatives or friends, 1.81% (3) to colleagues, 24.10% (40) to someone during social activities, and 22.29% (37) to someone during travelling in regions with severe epidemic of COVID-2019, respectively. For the 2 478 people exposed to the 22 epidemic cluster settings, the overall attack rate 6.70%. For the 41 families involved in the epidemic clusters, the secondary family attack rate was 57.27% (87/152). For all the cases of the clusters, the estimated incubation period ranged from 2 to 21 days, with an average of 7.11 days, a median of 5 days and a mode of 3 days; the estimated generation interval ranged from 1 to 21 days, with an average of 7.0 days, a median of 6 days, and a mode of 3 days, respectively.
Conclusion Family clustering is a main prevalent pattern of 2019 novel conronavirus disease epidemic in Ningbo city; the infections during incubation period and asymptomatic infections can give rise to viral transmission in COVID-2019 epidemics.
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