Figure 1. Structure of compartments used in modelling.

A susceptible individual in contact with a person: (1) presymptomatic, (2) who developed mild symptoms, or (3) who developed severe symptoms, may contract the infection at rate β, β or rβ*β, respectively, and enter the latent non-infectious compartment where he is infected but not yet infectious. During the non-infectious period, each individual has a probability of niε of becoming presymptomatic infectious. The presymptomatic cases have probability iε of developing severe or mild COVID-19 symptoms, with probabilities pS and 1−pS, respectively. The transition from both symptomatic groups occurs at μ⁻¹ rate. Individuals who developed severe symptoms do not travel within and between modelled subpopulations and may be either diagnosed with probability pDS, or recover with probability of 1−pDS. Individuals whose mild (or non-existent) symptoms are not stopping them from travelling may be diagnosed with probability pDM or recover with probability 1−pDM. The diagnosed individuals are considered isolated and effectively non-contagious and recover with rate μ⁻¹. The recovery does not discriminate between true recovery and fatal cases.