Spatial Dynamics of Australia's Syphilis Surge: Geographic Clustering, Service Accessibility, and the Dual Epidemic, 2011 to 2024

BackgroundAustralia is experiencing its worst syphilis epidemic in decades, driven by two distinct outbreaks -- a heterosexual epidemic disproportionately affecting Aboriginal and Torres Strait Islander communities in northern Australia, and a predominantly MSM-associated epidemic in urban centres. We examined the spatial dynamics of this dual epidemic, including geographic clustering, service accessibility associations, and COVID-19 impacts. MethodsWe analysed publicly available aggregate surveillance data (2013-2024). Spatial autocorrelation (Global/Local Morans I, Getis-Ord Gi*), spatial regression (OLS, spatial lag, spatial error), and geographically weighted regression (GWR) were applied to 74-75 Victorian local government areas (LGAs). Congenital syphilis vulnerability was mapped at SA3 level using composite proxy scoring. COVID-19 impact was assessed using interrupted time series (ITS) analysis. ResultsNational notifications rose 3.5-fold from 1,719 (2013) to 6,036 (2022). Strong spatial clustering was identified (Morans I=0.560, p<0.001), with 11 high-high clusters in inner Melbourne. The spatial lag model identified distance to sexual health clinic (coefficient: -0.225, p=0.050) and Indigenous population proportion (coefficient: 0.210, p=0.045) as significant predictors. Eleven SA3 areas were classified as high vulnerability for congenital syphilis, predominantly in remote northern Australia. Monthly NSW ITS found a non-significant 6% reduction in notifications during COVID restrictions (IRR=0.94, p=0.123). Comparing the COVID period (2020-2021) with pre-COVID means, remote area notifications increased 91% and female notifications increased 108% compared with 45% for males, reflecting the steep underlying epidemic trajectory. ConclusionsAustralias syphilis epidemic exhibits distinct spatial patterns shaped by service accessibility and sociodemographic factors. The concentration of high-vulnerability areas in remote northern Australia highlights the need for targeted service expansion, point-of-care testing, and culturally appropriate outreach. The acceleration of heterosexual transmission signals increasing congenital syphilis risk requiring urgent antenatal screening strengthening.