The pluses and minuses of ℛ0

Abstract

The concept of the basic reproduction number (ℛ0) occupies a central place in epidemic theory. The value of ℛ0 determines the proportion of the population that becomes infected over the course of a (modelled) epidemic. In many models, (i) an endemic infection can persist only if ℛ0 > 1, (ii) the value of ℛ0 provides a direct measure of the control effort required to eliminate the infection, and (iii) pathogens evolve to maximize their value of ℛ0. These three statements are not universally true. In this paper, some exceptions to them are discussed, based on the extensions of the SIR model. © 2007 The Royal Society.