Assessing disparity in the distribution of HIV and sexually transmitted infections in Australia: a retrospective cross-sectional study using Gini coefficients.

Introduction: The risk of HIV and sexually transmitted infections (STIs) varies substantially across population groups in Australia. We examined this disparity in HIV/STI distribution using Gini coefficients, where scores closer to one indicate greater disparity. Methods: We used demographic and sexual behaviour data from the Melbourne Sexual Health Centre, between 2015 and 2018. We examined 88 642 HIV consultations, 92 291 syphilis consultations, 97 473 gonorrhoea consultations and 115 845 chlamydia consultations. We applied a machine learning-based risk assessment tool, MySTIRisk, to determine the risk scores. Based on individuals' risk scores and HIV/STIs diagnoses, we calculated the Gini coefficients for these infections for different subgroups. Results: Overall, Gini coefficients were highest for syphilis (0.60, 95% CI 0.57 to 0.64) followed by HIV (0.57, 95% CI 0.52 to 0.62), gonorrhoea (0.38, 95% CI 0.36 to 0.42) and chlamydia (0.31, 95% CI 0.28 to 0.35). Gay, bisexual and other men who have sex with men (GBMSM) had lower Gini coefficients compared with heterosexual men or women; HIV (0.54 vs 0.94 vs 0.96), syphilis (0.50 vs 0.86 vs 0.93), gonorrhoea (0.24 vs 0.57 vs 0.57) and chlamydia (0.23 vs 0.42 vs 0.40), respectively. The Gini coefficient was lower among 25-34 years than in other age groups for HIV (0.66 vs 0.83-0.90) and gonorrhoea (0.38 vs 0.43-0.47). For syphilis, the oldest age group (≥45 years) had a lower Gini coefficient than 18-24 years (0.61 vs 0.70). Conclusions: Our study demonstrated that HIV/STIs are more evenly distributed among GBMSM, suggesting widely disseminated interventions for GBMSM communities. In contrast, interventions for heterosexual men and women should be more targeted at individuals with higher risk scores.