Abstract
The SIS model of Hadeler and Castillo-Chavez [9] with a constant transfer rate of susceptible into a partially protected state has been modified to take into account vaccination at birth. The model shows backward bifurcation (existence of multiple endemic stationary states) for certain values of parameters. Parameter values ensuring the existence and nonexistence of endemic equilibria have been discussed. Local and global stability of equilibria have been investigated. The minimum effort required to eradicated the infection has been determined.