Background Syphilis control remains a challenge in many high-income countries, including Australia, where diagnoses are concentrated among gay, bisexual men and other men who have sex with men (GBM). The aim of this study is to project the syphilis epidemic among GBM under a range of scenarios.

A dynamic coinfection model of HIV and syphilis transmission among GBM in Victoria, Australia, was parametrised to test data from clinics in Melbourne and syphilis case notifications in Victoria. Projected outcomes were new syphilis infections between 2018 and 2025 under seven testing and behaviour change scenarios.

Among HIV-negative GBM, the model estimated that increasing syphilis testing coverage (69% - 75%) and frequency (~8-monthly - 6-monthly) could prevent 5% and 13% of syphilis cases respectively between 2018 and 2025 compared to the status quo. Among HIV-positive GBM, less syphilis testing due to changes in HIV care increased syphilis cases by 29% between 2018 and 2025 compared to the status quo. Under a scenario of 20% HIV pre-exposure prophylaxis (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 compared to the status quo, driven by increased testing among PrEP users.

The present study 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.