Transmission roles of symptomatic and asymptomatic COVID-19 cases: a modelling study

Abstract

Coronavirus disease 2019 (COVID-19) asymptomatic cases are hard to identify, impeding transmissibility estimation. The value of COVID-19 transmissibility is worth further elucidation for key assumptions in further modelling studies. Through a population-based surveillance network, we collected data on 1342 confirmed cases with a 90-days follow-up for all asymptomatic cases. An age-stratified compartmental model containing contact information was built to estimate the transmissibility of symptomatic and asymptomatic COVID-19 cases. The difference in transmissibility of a symptomatic and asymptomatic case depended on age and was most distinct for the middle-age groups. The asymptomatic cases had a 66.7% lower transmissibility rate than symptomatic cases, and 74.1% (95% CI 65.9-80.7) of all asymptomatic cases were missed in detection. The average proportion of asymptomatic cases was 28.2% (95% CI 23.0-34.6). Simulation demonstrated that the burden of asymptomatic transmission increased as the epidemic continued and could potentially dominate total transmission. The transmissibility of asymptomatic COVID-19 cases is high and asymptomatic COVID-19 cases play a significant role in outbreaks.

Keywords: Age-dependent contact; COVID-19; SARS-CoV-2; asymptomatic case; transmission.

Fig. 1.

Compartmental model for SARS-CoV-2 transmission, where ‘j’ represents another age group different from ‘i’ for the compartments.

Fig. 2.

(a) The estimated transmissibility and 95% credible intervals for each age group; (b) The ratios of asymptomatic transmissibility to symptomatic transmissibility for seven age groups.

Fig. 3.

The proportion of asymptomatic infections and unconfirmed asymptomatic infections until 22 February 2020, for seven age groups. The estimated proportions of asymptomatic cases, the proportions of cases that failed to be detected among asymptomatic infections (unconfirmed proportions), and the observed proportions of asymptomatic cases are defined as: , and , respectively. The 95% credible intervals for the estimated proportions of asymptomatic cases are shown for each age group.

Fig. 4.

The estimated dynamics of the epidemic and the transmission burdens from symptomatic and asymptomatic cases. (a): The estimated numbers of daily new transmissions with 95% credible intervals and the observed numbers of daily reported new confirmed cases from 8 January to 22 February 2020; (b) The observed numbers of daily reported new confirmed symptomatic (R_cs) and asymptomatic cases (R_ca) and the estimated numbers of daily new cases that failed to be detected (R_h) with 95% credible intervals; (c): The estimated numbers of infected individuals caused by symptomatic and asymptomatic transmission over time, with 95% credible intervals; (d): Corresponding proportions of symptomatic and asymptomatic transmissions over different time periods.

Fig. 5.

The burden of transmission caused by different ages. (a) The estimated (contribution) ratios of new transmissions from different ages over different time periods; (b) The estimated (contribution) ratios of symptomatic and asymptomatic transmission from different ages. The contribution ratio of each age group is calculated by the proportion of the transmissions caused by the corresponding age group to the number of all transmissions in each transmission type, from 8 January to 1 February 2020.