Background: Syphilis control remains a challenge in many high-income countries, including Australia where diagnoses are concentrated among gay and bisexual men (GBM). We aimed to estimate the syphilis epidemic among GBM and project new cases under a range of intervention scenarios. Methods and materials: We parametrised a dynamic coinfection model of HIV and syphilis transmission among GBM in Victoria, Australia to testing data from high caseload clinics in Melbourne and Victorian syphilis case notifications. Projected outcomes were the annual new number of new syphilis infections 2018-2025 under seven testing coverage, frequency and behavior change scenarios, including reductions in the frequency of HIV care and HIV pre-exposure prophylaxis (PrEP) scale-up. Results: Among HIV-negative GBM, the model estimated that increasing syphilis testing coverage (69% to 75%) and frequency (~8-monthly to 6-monthly) from baseline was projected to prevent 5% and 13% of syphilis cases respectively between 2018-2025, or 18% of cases when combined. Among HIV-positive GBM, less syphilis testing associated with less frequent visits for HIV care was projected to increase syphilis cases by 29% between 2018 and 2025. Under a scenario of 20% HIV PrEP coverage among HIV-negative GBM (and associated increased serodiscordant sex, reduced condom use, and increased syphilis testing), syphilis cases were estimated to decrease by 6% among HIV-negative GBM and by 3% among HIV-positive GBM, as a result of increased testing among PrEP users. Conclusion: Our findings support syphilis control policies focusing on increased testing among GBM. Current Australian PrEP guidelines of quarterly syphilis testing are likely to negate any increases in syphilis due to risk compensation occurring with PrEP scale-up.
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