Visualizing the invisible: The effect of asymptomatic transmission on the outbreak dynamics of COVID-19

Abstract

Understanding the outbreak dynamics of the COVID-19 pandemic has important implications for successful containment and mitigation strategies. Recent studies suggest that the population prevalence of SARS-CoV-2 antibodies, a proxy for the number of asymptomatic cases, could be an order of magnitude larger than expected from the number of reported symptomatic cases. Knowing the precise prevalence and contagiousness of asymptomatic transmission is critical to estimate the overall dimension and pandemic potential of COVID-19. However, at this stage, the effect of the asymptomatic population, its size, and its outbreak dynamics remain largely unknown. Here we use reported symptomatic case data in conjunction with antibody seroprevalence studies, a mathematical epidemiology model, and a Bayesian framework to infer the epidemiological characteristics of COVID-19. Our model computes, in real time, the time-varying contact rate of the outbreak, and projects the temporal evolution and credible intervals of the effective reproduction number and the symptomatic, asymptomatic, and recovered populations. Our study quantifies the sensitivity of the outbreak dynamics of COVID-19 to three parameters: the effective reproduction number, the ratio between the symptomatic and asymptomatic populations, and the infectious periods of both groups For nine distinct locations, our model estimates the fraction of the population that has been infected and recovered by Jun 15, 2020 to 24.15% (95% CI: 20.48%-28.14%) for Heinsberg (NRW, Germany), 2.40% (95% CI: 2.09%-2.76%) for Ada County (ID, USA), 46.19% (95% CI: 45.81%-46.60%) for New York City (NY, USA), 11.26% (95% CI: 7.21%-16.03%) for Santa Clara County (CA, USA), 3.09% (95% CI: 2.27%-4.03%) for Denmark, 12.35% (95% CI: 10.03%-15.18%) for Geneva Canton (Switzerland), 5.24% (95% CI: 4.84%-5.70%) for the Netherlands, 1.53% (95% CI: 0.76%-2.62%) for Rio Grande do Sul (Brazil), and 5.32% (95% CI: 4.77%-5.93%) for Belgium. Our method traces the initial outbreak date in Santa Clara County back to January 20, 2020 (95% CI: December 29, 2019 - February 13, 2020). Our results could significantly change our understanding and management of the COVID-19 pandemic: A large asymptomatic population will make isolation, containment, and tracing of individual cases challenging. Instead, managing community transmission through increasing population awareness, promoting physical distancing, and encouraging behavioral changes could become more relevant.

The reproduction number of COVID-19 and its correlation with public health interventions

“…This value is lower than 94% for the measles, 89% for chickenpox with, 86% for mumps and rubella, and 80% for polio [ 3 ], but significantly higher than the values of 16% to 27% for the seasonal flu [ 7 ]. The countries with the highest prevalence, Luxembourg with 0.72%, Sweden with 0.71%, and Spain with 0.64% [ 15 ], do currently not even come close to these values, not even when including asymptomatic cases that are believed to increase the prevalence by an order of magnitude [ 43 ], resulting in 7.2%, 7.1%, and 6.4%. Knowing the precise basic reproduction number of COVID-19 will be critical to estimate the conditions for herd immunity and predict the success of vaccination strategies.…”

Is it safe to lift COVID-19 travel bans? The Newfoundland story

“…To account for societal and political actions [ 39 ], we introduce a behavior specific dynamic contact rate β = β ( t ) that varies both in space and time [ 22 ]. For easier interpretation, we express the contact rate, in terms of the dynamic effective reproduction number R( t ) [ 9 ], for which we make an ansatz of Gaussian random walk type [ 30 ] with a constant time-window of four days, Here N( t ) is the time-varying Gaussian distribution, parameterized in terms of the drift μ and the daily stepwidth τ = τ */[1.0− s ], where τ * is the the step width precision and s is the associated smoothing parameter [ 23 ].…”

The impact of asymptomatic COVID-19 infections on future pandemic waves

“…This poses a particular risk as schools and universities come back to campus in the fall of 2020. Second, high levels of undocumented cases may imply that cities are closer to achieving herd immunity than suggested by confirmed case counts [25][26][27] .…”