Objectives The number of incident infections of syphilis and HIV have increased over the past decade across Australia, particularly among gay men. In other industrialised settings, syphilis epidemics have also resurged coincidentally with increases in HIV diagnoses. Sexually transmissible infections (STI) are a biologically plausible cofactor for increasing HIV transmission. We pose the question: could strategies purely targeting syphilis also have an indirect impact on HIV incidence? Methods: We developed an agent-based computer model that simulates the transmission and disease progression of HIV and syphilis among a population of sexually active gay men, calibrated to reflect the epidemics in Victoria, Australia. The model was informed by detailed behavioural data from a variety of sources and was used to investigate the potential epidemiological impact of different public health interventions. Results: Assuming that syphilis could act as a biological cofactor for HIV transmission, from no effect to increasing risk by five-fold, our model indicates that if Australia’s syphilis action plan is effectively implemented then the number of HIV infections could decrease by up to 48% over the next decade in the absence of any specific HIV interventions. Conclusion: It is plausible that effective implementation of interventions targeting syphilis epidemics can have an indirect effect of mitigating the spread of HIV. The possible effects of STI should be considered in the design, implementation and evaluation of public health strategies and programs.
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