The time scale of asymptomatic transmission affects estimates of epidemic potential in the COVID-19 outbreak

Abstract

The role of asymptomatic carriers in transmission poses challenges for control of the COVID-19 pandemic. Study of asymptomatic transmission and implications for surveillance and disease burden are ongoing, but there has been little study of the implications of asymptomatic transmission on dynamics of disease. We use a mathematical framework to evaluate expected effects of asymptomatic transmission on the basic reproduction number R₀ (i.e., the expected number of secondary cases generated by an average primary case in a fully susceptible population) and the fraction of new secondary cases attributable to asymptomatic individuals. If the generation-interval distribution of asymptomatic transmission differs from that of symptomatic transmission, then estimates of the basic reproduction number which do not explicitly account for asymptomatic cases may be systematically biased. Specifically, if asymptomatic cases have a shorter generation interval than symptomatic cases, R₀ will be over-estimated, and if they have a longer generation interval, R₀ will be under-estimated. Estimates of the realized proportion of asymptomatic transmission during the exponential phase also depend on asymptomatic generation intervals. Our analysis shows that understanding the temporal course of asymptomatic transmission can be important for assessing the importance of this route of transmission, and for disease dynamics. This provides an additional motivation for investigating both the importance and relative duration of asymptomatic transmission.

Keywords: Asymptomatic transmission; Basic reproduction number; COVID-19; Coronavirus disease; SARS-CoV-2.

Fig. 1

Effects of intrinsic proportion of asymptomatic transmission on the realized proportion of asymptomatic transmission and basic reproduction number, given variation in the mean generation interval of asymptomatic cases. (A) Increasing the speed of asymptomatic transmission (shorter generation intervals) increases the realized proportion of asymptomatic transmission, q. (B) Increasing the speed of asymptomatic transmission (shorter generation intervals) decreases the basic reproduction number ${\mathcal{R}}_0$. When ${\overline{G}}_a$ is smaller (larger) than ${\overline{G}}_s$, estimates based on the observed generation distribution for symptomatic cases (${\mathcal{R}}_0=2.5$; dashed line) are expected to over- (under-) estimate the true ${\mathcal{R}}_0$. For both panels, the circle denotes z = 0.5 and ${\overline{G}}_a/{\overline{G}}_s=0.55$ whereas the triangle denotes z = 0.5 and ${\overline{G}}_a/{\overline{G}}_s=1.8$. Solid lines show contours for q and ${\mathcal{R}}_0$ values. The dashed line represents the naive estimate that assumes ${\overline{G}}_a={\overline{G}}_s$. Here, we assume 1/r = 7 days, ${\overline{G}}_s=8$ days, and κ_s = κ_a = 0.5.