Efficiency Evaluation on Case Finding Strategy for COVID-19 Outbreak Control under China's "Dynamic Zero-Case Policy": a Retrospective Field Epidemiology Study

Zhibei Zheng¹Wanwan Sun²Shelan Liu²Enfu Chen²Jinren PAN^2*

• ¹Hangzhou Center for Disease Control and Prevention (HZCDC), Hangzhou, Zhejiang Province, China
• ²Zhejiang Center for Disease Control and Prevention (Zhejiang CDC), Hangzhou, China

Objectives: To evaluate the efficiency of case-finding strategy for COVID-19 outbreak control during the "dynamic zero-case policy" period in Zhejiang Province, China, in 2022.A field epidemiological observational study was conducted to describe the proportion and time distribution of all cases identified in the event. Categorical data were expressed as counts/proportions or positive rates. The proportion of control lead-time was developed to evaluate the performance of management in various at-risk populations. Positivity rates were used to evaluate the efficiency of certain case-finding approaches. The Pearson χ² test was used to compare proportions between the groups.Results: Close-contact tracing identified 62.3% of the total cases. Low-risk area resident screening revealed 15.2% of the cases, most of which were ascertained within the first 3 days. All cases found by second-layer contact tracing had a longer control lead-time, but transmission events were believed to occur during the transportation or quarantine period. A higher proportion of positive control leadtime was more effective in curbing SARS-CoV-2 transmission on subsequent days. The proportion of control lead-time between those with and without quarantine status was statistically different (χ² (1) = 248.5, P<0.001). 442 cases (0.571%) were found out of 77 462 close contacts, while 9 cases (0.104%) were found out of 8 683 second-layer contacts (χ² (1) = 32.7, P<0.001). On average, 2.2, 7.4, and 106.5 cases were detected per million tests by low-, medium-, and high-risk area residents, respectively.Conclusions: Rapid and stringent comprehensive public health and social measures can contain the spread of SARS-CoV-2 in a localized area within weeks. Close-contact tracing plays a pivotal role in COVID-19 outbreak control, but contact tracing alone is insufficient to contain the transmission. Mass screening in the early stage and outbreak site exposure person tracing also play an important role for cases finding. It is suggested that in containing severe acute infections with direct transmission route in the future, if stringent social distancing requirements have already being implemented, measures such as tracing second-layer contacts, repeated mass screening for mediumor low-risk residents during the middle and late stages of the event are not recommended.