Evaluating strategies to combat a major syphilis outbreak in Australia among Aboriginal and Torres Strait Islander peoples in remote and regional Australia through mathematical modelling.

Hui, B. B.; Ward, J. S.; Guy, R.; Law, M. G.; Gray, R. T.; Regan, D. G.

Background An ongoing infectious syphilis outbreak, first reported among Australian Aboriginal and Torres Strait Islander people in 2011, has resulted in >3000 notifications to the end of 2019 with multiple congenital syphilis cases and infant deaths. In 2017, the Australian Government introduced an enhanced test and treat response. We evaluate the impact of this response and the potential impact of further expansion of testing interventions. Methods We developed a mathematical model to capture the transmission of infectious syphilis among young heterosexual Indigenous Australians aged 15-29 years living in regional and remote areas. We used the model to assess the impact of existing and hypothetical outbreak responses on infectious syphilis prevalence. Findings The increase in testing coverage achieved through the enhanced response (18% coverage in 2011, to 39% in 2019) could lead to a stabilisation of the epidemic from 2021. To return to the pre-outbreak level (<0.2%) within five years, testing coverage needs to reach 60%. With the addition of a biannual community-wide screening program, using outreach to test 30% of youth in communities over 6 weeks, a return to pre-outbreak levels can be achieved within 2 years. If testing coverage alone was scaled-up to 60% at the start of outbreak in 2011, syphilis prevalence would have returned to pre-outbreak levels by 2014. Interpretation Modelling suggests that to control the syphilis outbreak the response needs to be delivered with further potency. The reduction in prevalence could be hastened with community-wide screening at similar time periods across all communities along with increases in annual testing coverage.