Infectivity And Influencing Factors Associated With SARS-CoV-2 Transmission Under Contact Tracing

Objective:By collecting data on the cohort of index cases-close contacts,analyze the infectivity of symptomatic cases and asymptomatic cases,the initial clinical manifestations and clinical severity of secondary cases,and explore the influencing factors of infectivity and severity,to provide scientific basis for the identification,tracking,management of close contacts and the early identification of cases.Methods:Based on a retrospective cohort survey,this study collected information of COVID-19 cases(including symptomatic cases and asymptomatic cases)and their close contacts between January 5 and April 7,2020 in five regions(Hubei,Jiangsu,Zhejiang,Guangdong and Chongqing).Descriptive epidemiology was used to analyze the characteristics of index cases and close contacts,the initial clinical manifestations and spectrum of infection of secondary cases.Compare the secondary infection rate(SIR)of different characteristics to analyze the infectivity of SARS-CoV-2;and use multivariate logistic regression model explores the influencing factors of SARS-CoV-2 infectivity and severity.Results:1.This study included 1005 index cases(including 376 asymptomatic cases and 629 symptomatic cases)and 7243 close contacts confirmed from January 5 to April 7,2020.There were 5281 close contacts of 629 symptomatic cases,and 278 were infected with SARS-CoV-2,with a SIR of 5.26%(95%CI:4.68-5.90).1962 close contacts of 376 asymptomatic cases,and 32 were infected with SARS-CoV-2,with a SIR of 1.63%(95%CI:1.12-2.29).The SIR of symptomatic cases was higher than that of asymptomatic cases,with statistical significance(P<0.001).16.92%of index cases caused the infection of the secondary case,and 83.08%of index cases did not cause the infection of their close contacts.And an average of 7.2 close contacts were traced to each index case.The average number of secondary cases caused by each index case was 0.31.2.310 of the 7243 close contacts were infected,and the total secondary infection rate was 4.28%(95%CI:3.83-4.77).Hubei(4.70%,95%CI:3.84-5.69)and Chongqing(3.47%,95%CI:2.70-4.38)had higher SIR.The SIR of contact through living together and sharing meal were 7.62%(95%CI:6.69-8.63)and 5.22%(95%CI:3.76-7.04).The SIR in daily communication,living in the same confined space,and sharing transportation were 1.16%(95%CI:0.55-1.86),0.48%(95%CI:0.06-1.73)and 0.15%(95%CI:0.00-0.86),respectively.SIR of severe/critical cases(6.48%,95%CI:4.71-8.66)was higher than that of mild/moderate cases(5.09%,95%CI:4.48-5.77).Multivariate logistic regression analysis of the infectivity of symptomatic cases showed that contact way(multiple exposure vs daily conversation,OR=6.11,95%CI:2.95-13.5;living together vs daily conversation,OR=5.46,95%CI:3.02-10.9;sharing meal vs daily conversation,OR=3.20,95%CI:1.63-6.76),exposure period(symptomatic period vs incubation period,OR=1.79,95%CI:1.19-2.76),sex(female vs male,OR=1.31,95%CI:1.01-1.69),age(20-59 years vs<20 years,OR=1.73,95%CI:1.19-2.58;≥60 years vs<20 years,OR=1.82,95%CI:1.15-2.91),underlying disease(yes vs no,OR=3.41,95%CI:2.27-5.05),and exposure level(≥2 symptomatic cases vs 1 symptomatic case,OR=3.17,95%CI:2.42-4.17;exposure to both symptomatic and asymptomatic cases vs 1 symptomatic case,OR=2.30,95%CI:1.21-4.11)were influencing factors for the infectivity of symptomatic cases.Multivariate logistic regression analysis of the infectivity of asymptomatic cases showed that contact way(sharing meal vs daily conversation,OR=8.39,95%CI:1.71-60.6)and sex(female vs male,OR=2.27,95%CI:1.10-4.95)were the influencing factors for the infectivity of asymptomatic cases.3.Among the secondary cases,asymptomatic cases accounted for 28.71%and symptomatic cases accounted for 71.29%.Of them,mild cases accounted for 20.00%,moderate cases accounted for 40.97%,severe cases accounted for 6.77%,critical cases accounted for 1.29%,and fatal cases accounted for 2.26%.The age of asymptomatic cases was younger than that of symptomatic cases(36 years vs 49 years,P<0.001),and the proportion of underlying disease was lower(8.99%vs 19.91%,χ2=4.666,P=0.031).The proportion of asymptomatic cases in secondary cases caused by asymptomatic cases was higher than that of symptomatic cases(73.68%vs 22.92%,χ2=21.08,P<0.001).The proportion of mild/moderate cases caused by symptomatic cases was higher than that of asymptomatic cases(22.22%vs 5.26%,χ2=12.57,P<0.001).Among the 221 symptomatic cases,the common initial symptoms were fever(49.77%),cough(42.53%),fatigue(14.03%)and diarrhea(8.60%).4.Among the secondary cases,severe/critical cases accounted for 10.32%.Multivariate logistic regression model showed that age(≥60 years 20-59 years,OR=11.3,95%CI:4.66-30.8)and underlying disease(yes vs no,OR=2.67,95%CI:1.06-6.70)were influencing factors for severe COVID-19.Conclusions:(1)The infectivity of asymptomatic cases is lower than that of symptomatic cases.COVID-19 cases are infectious during the incubation period and have a high risk of transmission during the symptomatic period.Most secondary transmission may be caused by a small number of cases.(2)Living together,sharing meal,multiple contact ways,exposure to symptomatic period,female,elderly,with underlying diseases,and exposure to multiple index cases are risk factors for higher infectivity of COVID-19.(3)The clinical outcome of asymptomatic cases with secondary cases is less severe than that of symptomatic cases.Asymptomatic cases are younger than symptomatic cases and have fewer underlying diseases.The secondary symptomatic cases had a wide range of initial clinical symptoms,mainly fever,cough,fatigue,diarrhea,etc.(4)Old age and underlying diseases are risk factors for severe COVID-19.