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
Authors: Hui Ben B. , Ward James S. , Guy Rebecca , Law Matthew G. , Gray Richard T. , Regan David G. 
Published: March 2021

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.

Funding

The research was undertaken by the Kirby Institute, UNSW Sydney, for the Multi-jurisdictional Syphilis Outbreak Working Group (MJSO) with funding from the Australian Department of Health.

Research in context

Evidence before this study

We search PubMed with the terms ((“syphilis”[MeSH Terms] OR “syphilis”[All Fields]) AND (“disease outbreaks”[MeSH Terms] OR (“disease”[All Fields] AND “outbreaks”[All Fields]) OR “disease outbreaks”[All Fields] OR “outbreak”[All Fields])) AND “model “[All Fields]) on 12 March 2020 and identified 27 articles. Most articles focused on men who have sex with men and/or populations with HIV co-infection, neither of which is common in our target population (Indigenous Australians: predominantly aged 15-29; heterosexual and living in regional and remote areas). Of the remaining articles that consider syphilis control through screening interventions, the most relevant paper to our study is a modelling paper by Pourbohloul et al. in 2003, which demonstrated that community-wide treatment has no lasting effect on syphilis transmission.

Added value of this study

We developed a mathematical model to assess the impact of an enhanced response to a major syphilis outbreak in remote Aboriginal and Torres Strait Islander populations of Australia and whether it can be controlled by increasing testing coverage. The model captures sexual behaviour information and short-term population mobility patterns across regional and remote communities of Australia and was calibrated against the most recent infectious syphilis notification data to-date (up to 2019) and testing coverage data from the affected regions. Our findings provide an insight into the role of increasing testing coverage in controlling syphilis outbreaks among populations living in remote communities globally.

Implications of all the available evidence

Existing evidence suggests that enhanced surveillance, expanded clinical and laboratory services, enhanced health promotion, strengthened community involvement and a rapid outbreak response are core components to controlling syphilis outbreaks. Our study focuses on modelling the impact of expanded clinical services and ability to expand testing among the target population. Our study suggests that increasing testing coverage of the Aboriginal and Torres Strait Islander population aged 15-29 living in remote and regional communities to a level of 60%, would stabilise the epidemic and reduce overall prevalence to pre outbreak levels in around five years. Combining the 60% testing coverage with bi-annual community screening over a 6-week period involving outreach (minimum coverage of 30%) would reduce the time period to around 2 years.

We believe these findings have implications for other Indigenous populations across the world who often live in remote regions with limited access to healthcare and are disproportionately affected by STIs.