Objective: To investigate the infection rate in close contacts of COVID-19 patients before and after the last negative nucleic acid test, evaluate the effect of dynamic nucleic acid test in determining the infectivity of COVID-19 patients. Methods: Dynamic nucleic acid test results of COVID-19 cases were collected in a retrospective cohort study. COVID-19 cases with negative nucleic acid test results before their first positive nucleic acid tests were selected as study subjects. Close contacts of the index cases and the secondary close contacts were kept isolation for medical observation to assess their risk of infection. Results: This study included 89 confirmed cases from two local COVID-19 epidemics in Ningbo. A total of 5 609 close contacts were surveyed, the overall infection rate was 0.20%. No close contacts of the COVID-19 cases before the last negative nucleic acid test were infected, and the infection rate in the close contacts of the COVID-19 cases after the last negative nucleic acid test was 1.33%, all of these close contacts lived together with the index cases. No secondary close contacts were infected. Conclusion: COVID-19 patient becomes infectious after the last nucleic acid is negative, and has no infectivity before the last nucleic acid negative.
目的： 研究新型冠状病毒（新冠病毒）感染者末次核酸阴性前后密切接触者（密接）的感染率，评估动态核酸检测结果在判断新冠病毒感染者传染性中的作用。 方法： 采用回顾性队列研究的方法，收集新冠病毒感染者的动态核酸检测结果。选择新冠病毒感染者在首次核酸检测阳性前有核酸阴性结果者为研究对象，对感染者的密接以及密接的密接（次密接）进行持续隔离医学观察，评估密接和次密接的发病风险。 结果： 共纳入宁波市2起本土疫情的89例新冠病毒感染者，均为确诊病例。追踪调查了5 609名密接，密接的总感染率为0.20%。感染者末次核酸阴性前的密接无感染，末次核酸阴性后的密接感染率为1.33%，且均为与指示病例共同居住生活者。感染者涉及的所有次密接均未感染。 结论： 新冠病毒感染者末次核酸阴性前无传染性，末次核酸阴性后开始具有传染性。.