A SEIR-like model with a time-dependent contagion factor describes the dynamics of the Covid-19 pandemic

I consider a simple, deterministic SEIR-like model without spatial or age structure, including a presymptomatic state and distinguishing between reported and nonreported infected individuals. Using a time-dependent contagion factor β ( t ) (in the form a piecewise constant function) and literature values for other epidemiological parameters, I obtain good fits to observational data for the cumulative number of confirmed cases in over 160 regions (103 countries, 24 Brazilian states and 34 U.S. counties). The evolution of β is useful for characterizing the state of the epidemic. The analysis provides insight into general trends associated with the pandemic, such as the tendency toward reduced contagion, and the fraction of the population exposed to the virus.