一起由Omicron变异株BA.5.2引起的义乌市新冠病毒感染疫情传播特征分析

Transmission characteristics of a COVID-19 outbreak caused by Omicron variant BA.5.2 in Yiwu city,China

目的对浙江省义乌市一起由Omicron变异株BA.5.2引起的新冠病毒感染疫情的传播特征进行分析。方法将2022年8月2日-8月21日义乌市新冠病毒感染疫情所涉及的感染者为研究对象,收集流行病学调查、实验室检测、临床症状等信息,分析家庭续发率、传染性代际(transmission generations,TG)、序列间隔(serial interval,SI)、感染者出现临床症状到核酸检测阳性间隔时间、实时再生数Rt等反映疫情传播特征指标。结果该起疫情为指示病例发病前7 d有外省高风险区域旅居史和外省阳性感染者的密切接触史后引入义乌市,基因测序显示病毒属于VOC/Omicron(BA.5.2进化分支)变异株。共报告713例感染者,疫情持续20 d。男性343例,女性370例。年龄M(P25,P75)为33(20,74)岁。义乌市除赤岸镇外的13个街道均有病例报告。家庭聚集性疫情138起(74.2%),涉及425例感染者;在传播关系明确的101起家庭聚集性疫情中,家庭续发率为62.2%(95%CI:56.5%~67.5%),家庭成员全部续发的家庭占39.6%(40/101),其中引入病例和197例续发病例TG的M(P25,P75)为1.7(1.0,2.8)d,TG≤2 d的感染者所占50.8%;在65起家庭聚集性疫情中,引入病例和99例续发病例均出现了临床症状,SI的M(P25,P75)为2.0(1.0,3.0)d,SI≤2 d的感染者占44.4%。开展个案流行病学调查时364例(51.1%)感染者自述出现临床症状,以发热(64.3%)、咳嗽(23.4%)、咽痛咽干(20.1%)为主。感染者出现临床症状到核酸检测阳性的时间间隔M(P25,P75)为0(0,1)d,124例(34.1%)在出现临床症状后检出核酸阳性,滞后时间以1 d为主(67.7%),其次为间隔2 d(22.6%)。未发现核酸滞后与非滞后组之间性别、年龄、采样方式、采样时间段、Ct值的差异。疫情初期时实时再生数(Rt)最高为6.18,在采取防控措施1周后(8月9日)降至1以下。结论本起由Omicron变异株BA.5.2引起的新冠病毒感染疫情传播特征表现为家庭续发率高,传染性代际和序列间隔短,且存在较高比例的核酸阳性滞后于症状感染者,隐匿性强。

Objective To analyze transmission characteristics of a coronavirus disease 2019(COVID-19)outbreak caused by Omicron variant BA.5.2 in Yiwu city of Zhejiang province,China.Methods Medical records and the information on epidemiological investigation,laboratory tests were collected for all local COVID-19 cases diagnosed during August 2–21,2022-a period of an COVID-19 outbreak in Yiwu city.The transmission characteristics of the outbreak was analyzed using indicators including family secondary attack rate,transmission generations(TG),serial interval(SI),the time interval from symptom onset to positive nucleic acid test,as well as time-varying reproduction number(Rt).Results The indicator case of the outbreak was confirmed with the history of living in high-risk area of other provinces and close contact with COVID-19 infected persons 7 days before the onset of disease symptoms.Genome sequencing of the virus isolated from the indicator case's secondary cases indicated that the viral strain of the outbreak was severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)Omicron variant BA.5.2.Totally 713 cases(343 males and 370 females)from 13 subdistricts(except for Chi'an township)of Yiwu city were confirmed during the outbreak lasting for 20 days.The median age of the cases(percentile 25[P25],percentile 75[P75])was 33(20,74)years.A total of 138 household clusters(involving 74.2%of all infections)were identified in the outbreak.For the 101 household clusters with clear transmission chain,the secondary attack rate was 62.2%(95%confidence interval:56.5%-67.5%);39.6%of the household clusters affected all family members of the households involved;the median TG(P25,P75)was 1.7(1.0,2.8)days for the primary cases and 197 secondary cases and the TG was equal or less than 2 days for 50.8%of the cases being involved.For 65 household clusters,the median SI(P25,P75)was 2.0(1.0,3.0)days for the primary cases and 99 secondary cases with clinic symptoms and the SI was equal or less than 2 days for 44.4%of the cases being involved.Of the 364 cases(51.1%of the cases undergoing epidemiological surveys)with self-reported symptoms,64.3%,23.4%,and 20.1%reported fever,cough,and sore/dry throat,respectively.For the 364 symptomatic cases,the median time interval(P25,P75)from symptom onset to positive nucleic acid test was 0(0,1)day and the time interval was one day for 67.7%of the cases and two days for 22.6%of the cases.No significant differences were observed in gender,age,site and daily time point of specimen sampling for nucleic acid test,and cycle threshold(Ct)value between the cases with and without lag time(day)of positive nucleic acid test from symptom onset.The maximum Rt was 6.18 in early period of the COVID-19 outbreak and the Rt decreased to less than one week after the implementation of containment measures.Conclusion The COVID-19 outbreak caused by Omicron variant BA.5.2 was characterized by high family secondary attack rate,short transmission generation and serial interval,and a high proportion of cases with lag time of positive nucleic acid test from symptom onset.