Extinction thresholds in deterministic and stochastic epidemic models

Abstract

The basic reproduction number, ℛ0, one of the most well-known thresholds in deterministic epidemic theory, predicts a disease outbreak if ℛ0>1. In stochastic epidemic theory, there are also thresholds that predict a major outbreak. In the case of a single infectious group, if ℛ0>1 and i infectious individuals are introduced into a susceptible population, then the probability of a major outbreak is approximately 1−(1/ℛ0) i . With multiple infectious groups from which the disease could emerge, this result no longer holds. Stochastic thresholds for multiple groups depend on the number of individuals within each group, i j , j=1, …, n, and on the probability of disease extinction for each group, q j . It follows from multitype branching processes that the probability of a major outbreak is approximately . In this investigation, we summarize some of the deterministic and stochastic threshold theory, illustrate how to calculate the stochastic thresholds, and derive some new relationships between the deterministic and stochastic thresholds.