Abstract
In this article, we present the continuing work on a SARS model without quarantine by Hsu and Hsieh [Sze-Bi Hsu, Ying-Hen Hsieh, Modeling intervention measures and severity-dependent public response during severe acute respiratory syndrome outbreak, SIAM J. Appl. Math. 66 (2006) 627-647]. An "acting basic reproductive number" ψ is used to predict the final size of the susceptible population. We find the relation among the final susceptible population size S <sub>∞</sub> , the initial susceptible population S <sub>0</sub> , and ψ. If ψ > 1, the disease will prevail and the final size of the susceptible, S <sub>∞</sub> , becomes zero; therefore, everyone in the population will be infected eventually. If ψ < 1, the disease dies out, and then S <sub>∞</sub> > 0 which means part of the population will never be infected. Also, when S <sub>∞</sub> > 0, S <sub>∞</sub> is increasing with respect to the initial susceptible population S <sub>0</sub> , and decreasing with respect to the acting basic reproductive number ψ. © 2006 Elsevier Inc. All rights reserved.