A New Epidemic Model for the COVID-19 Pandemic: The θ-SI(R)D Model

Since the beginning of the COVID-19 pandemic, a large number of epidemiological models have been developed. The principal objective of the present study is to provide a new six-compartment model for the COVID-19 pandemic, which takes into account both the possibility of re-infection and the differentiation between asymptomatic and symptomatic infected subjects. The model, denoted as θ-SI(R)D, is a six-compartment model, described by as many ordinary differential equations. The six compartments are denoted as Susceptible ((Formula presented.)), Symptomatic Infected ((Formula presented.)), Asymptomatic Infected ((Formula presented.)), Recovered from Asymptomatic fraction ((Formula presented.)), Recovered from Symptomatic fraction ((Formula presented.)), and Deceased ((Formula presented.)). Such a model has no analytical solutions, so we performed both a simulation and a model validation ((Formula presented.). Based on the results of our simulations (and, on the other hand, on the results of most of the models in the scientific literature), it is possible to draw the reasonable conclusion that the epidemic tends, even without vaccination, to a steady state.