The CDC launched the National Plan to Eliminate Syphilis from the USA in October 1999[4]. In order to reach this goal, a goodunderstanding of the transmission dynamics of the disease is necessary. Based on a SIRSmodel Breban et al. [3] providedsome evidence that supports the feasibility of the plan proving that no recurringoutbreaks should occur for syphilis. We study in this work a syphilis model that includespartial immunity and vaccination. This model suggests that a backward bifurcation verylikely occurs for the real-life estimated epidemiological parameters for syphilis. Thismay explain the resurgence of syphilis after mass treatment [21]. Occurrence of backward bifurcation brings a new challenge for theplan of the CDC’s –striking a balance between treatment of early infection, vaccinationdevelopment and health education. Our models suggest that the development of an effectivevaccine, as well as health education that leads to enhanced biological and behavioralprotection against infection in high-risk populations, are among the best ways to achievethe goal of elimination of syphilis in the USA.
