Fig. 2: Schematic of the SEIR-type model.
From: Swift and extensive Omicron outbreak in China after sudden exit from ‘zero-COVID’ policy
Our model maps the four epidemiological states-susceptible (S), exposed (E), infectious (I), recovered (R)-to the three states of the online survey. Individuals who report themselves as infected and recovered (green state) are only those who previously experienced symptoms (top track of states). Those reporting as infected with symptoms (red states) could be currently infectious or recovered but still symptomatic. All others report as not yet infected (purple states), whether they are susceptible, exposed, pre-symptomatic, or asymptomatic. People in all four infectious states can infect susceptibles, with transmission rate β. A proportion f of infected people eventually develop symptoms. Other parameters are rates of becoming infectious (kE), becoming symptomatic (kIP), resolving symptoms (kIS, kS), and recovering without symptoms (kA). Some epidemiological states are divided into substates (e.g., E1 and E2) to allow more realistic distributions of waiting times. The bottom track of states counts the fraction w of people who test positive with a delay governed by rate kT; note that this does not remove them from the epidemiological or survey states (thus with a dashed arrow). The corresponding system of ordinary differential equations, along with the rate parameter values, is provided in “Methods”.
