Risk population characteristic and influence of infections in a COVID-19 outbreak

Objective To describe the distribution and evaluate the control measures effect of the risk population types of infections in a local COVID-19 outbreak, which can deliver suggestions for another possible epidemic in the future.Methods All the epidemiological information of infections was collected for descriptive analysis. The risk population type of an infection was based on their management and control status when the first diagnosis of confirmed COVID-19 infection was made. The exponential growth rate was calculated by the incidence with 3-day moving average method. Univariate linear regression analysis was carried out between the number of infections detected on that day and total number of unquarantined infections in the period of communicability consecutive days before that day.Results A local COVID-19 outbreak was occurred in Yiwu city, Zhejiang Province in August 2022 with a total of 710 infections. The three largest cases number of detection ways were close-contacts tracing (62%), mass screening (15%), and outbreak focus sites exposure person screening (6%). Among the infections detected from close-contacts, 81% of the contact scenarios were living or working together with an index case, while respiratory specimen sampling, living in a same building, and visiting a same public place also accounted for a certain proportion. There was a strong negative correlation (r=-0.84) between the proportion of under-quarantine detection ways and the exponential growth rate. The linear regression fitted quite well (R-squared: 0.96) by the previous 3-day cumulative number of unquarantined infections in the period of communicability and the number of infections detected on that day. The detected numbers of infections were all fell on the prediction interval with 80% confidence level except for the first 3 days in this outbreak.Conclusions When carrying out containment strategy to deal with local outbreak, while attaching importance to isolation and treatment of infections and close-contacts tracing, it is necessary to strengthen the management of other risk population types like outbreak focus sites exposure person. Mass screening activity should be adopted prudently and well organized. It is shouldn’t adopt centralized quarantine for close-contacts’ close-contacts routinely when in the face of a larger outbreak. The proportion of under-quarantine detection ways can be used as a rapid assessment indicator of control strength, and the number of unquarantined infections in the period of communicability can be used to prejudge the epidemic trend.