Asymptomatic transmission in COVID-19 dynamics

Asymptomatic transmission has been critical to the spread of COVID-19, making the disease more difficult to control. However, the bulk of prior work assumes that asymptomatic transmission is fixed over time. Here we explore the coupling between asymptomatic incidence (at the level of individuals) with asymptomatic relevance (at the scale of populations). To do so, we build on prior work showing how the duration of asymptomatic vs. symptomatic transmission can influence the strength of COVID-19⁽¹⁾. Here, we explain an under-appreciated consequence of differences in the time-scale of transmission. We find that when asymptomatic individuals transmit for longer periods then the fraction of new cases attributed to asymptomatic transmission can decrease when total case counts are increasing and increase when total case counts are decreasing. Due to assortative mixing between age groups, we also show evidence that as case counts increase then the average age of those infected should go up, whereas as case counts decrease then the average age of those infected should go down. These results show the importance of the relative contribution of asymptomatic transmission toward overall disease dynamics in COVID-19.

⁽¹⁾Park, S. W., Cornforth, D. M., Dushoff, J., & Weitz, J. S. (2020). The time scale of asymptomatic transmission affects estimates of epidemic potential in the COVID-19 outbreak. Epidemics, 100392.