The CDC launched the National Plan to Eliminate Syphilis from the USA in October 1999
[4]. In order to reach this goal, a good
understanding of the transmission dynamics of the disease is necessary. Based on a SIRS
model Breban et al. [3] provided
some evidence that supports the feasibility of the plan proving that no recurring
outbreaks should occur for syphilis. We study in this work a syphilis model that includes
partial immunity and vaccination. This model suggests that a backward bifurcation very
likely occurs for the real-life estimated epidemiological parameters for syphilis. This
may explain the resurgence of syphilis after mass treatment [21]. Occurrence of backward bifurcation brings a new challenge for the
plan of the CDC’s –striking a balance between treatment of early infection, vaccination
development and health education. Our models suggest that the development of an effective
vaccine, as well as health education that leads to enhanced biological and behavioral
protection against infection in high-risk populations, are among the best ways to achieve
the goal of elimination of syphilis in the USA.