Infectivity, susceptibility, and risk factors associated with SARS-CoV-2 transmission under intensive contact tracing in Hunan, China

Abstract

Several mechanisms driving SARS-CoV-2 transmission remain unclear. Based on individual records of 1178 potential SARS-CoV-2 infectors and their 15,648 contacts in Hunan, China, we estimated key transmission parameters. The mean generation time was estimated to be 5.7 (median: 5.5, IQR: 4.5, 6.8) days, with infectiousness peaking 1.8 days before symptom onset, with 95% of transmission events occurring between 8.8 days before and 9.5 days after symptom onset. Most transmission events occurred during the pre-symptomatic phase (59.2%). SARS-CoV-2 susceptibility to infection increases with age, while transmissibility is not significantly different between age groups and between symptomatic and asymptomatic individuals. Contacts in households and exposure to first-generation cases are associated with higher odds of transmission. Our findings support the hypothesis that children can effectively transmit SARS-CoV-2 and highlight how pre-symptomatic and asymptomatic transmission can hinder control efforts.

Fig. 1. Temporal, geographical, and age distribution of SARS-CoV-2 infected individuals stratified by the presence of symptoms and source of infection in Hunan Province, China.

a Daily number of new SARS-CoV-2 infected individuals by date of symptom onset by source of infection. For asymptomatic individuals we used date of the first RT-PCR positive result. b Geographical distribution of SARS-CoV-2 infected individuals in Hunan Province, and geo-locations of Hubei and Hunan provinces. c Geographical distribution of SARS-CoV-2 clusters. d Age distribution of SARS-CoV-2 symptomatic and asymptomatic infected individuals.

Fig. 2. Quantifying the serial interval, infectiousness profile, incubation period, and generation time fitted by gamma or Weibull distributions.

a Estimated distribution of the serial interval and of the infectiousness profile by gamma distributions based on 245 transmission pairs. b Estimated distribution of the incubation period by Weibull distributions and of the generation time by gamma distributions based on 268 confirmed cases with locally acquired infection belonging to 114 clusters.

Fig. 3. Timing of transmission events and SARS-CoV-2 infected subjects in randomly selected clusters showing evidence of pre-symptomatic transmission.

Square symbols indicate symptomatic cases and circular symbols indicate asymptomatic subjects. Age, sex, and generation in a cluster are shown for each SARS-CoV-2 infected individual (left panels), with information on date of illness onset for symptomatic cases and date of the first RT-PCR positive result for asymptomatic subjects and for symptomatic cases without date of illness onset (symbol “*”). Timeline of events (right panels).