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

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.

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).

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.

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.