Emerging epidemiology of invasive group a Streptococcus in tropical Australia: a 21-year population-based data linkage study.

PURPOSE: Invasive Group A Streptococcus (iGAS) is an important cause of severe bacterial infection with increasing incidence and substantial socioeconomic disparities. This study quantified long-term trends, spatial distribution, and determinants of iGAS incidence in Northern Queensland, Australia. METHODS: A population-based retrospective cohort study was conducted using linked hospital data linkage from 2000-2020. Incidence rates were calculated per 100,000 person-years and temporal trends were assessed. Spatial clustering was evaluated using Moran's I statistic for spatial autocorrelation. Determinants of incidence were examined using a negative binomial generalized additive model (GAM). RESULTS: A total of 933 iGAS events were identified among 870 individuals. Incidence increased substantially over time, with the highest rates observed among older adults and a secondary peak among infants. Age-standardised incidence was approximately nine-fold higher among Indigenous compared with non-Indigenous populations for both males and females. Spatial analysis demonstrated significant clustering of incidence (Moran's I = 0.19, p < 0.001), with the greatest burden occurring in remote and very remote regions. In multivariable analysis, Indigenous status (IRR = 8.97, p < 0.001) and male sex (IRR = 1.28, p < 0.001) were independently associated with higher incidence. Smooth terms showed significant nonlinear effects of age (edf = 7.45, p < 0.001) and calendar year (edf = 8.04, p < 0.001), indicating heterogeneous age patterns and temporal acceleration in incidence. CONCLUSION: iGAS incidence in Northern Queensland increased markedly over two decades and was characterised by pronounced geographic and Indigenous health inequities, highlighting the need for targeted prevention strategies in high-risk populations and regions.