Applying population-specific incidence prevalence ratio benchmarks to monitor the Australian HIV epidemic: an epidemiological analysis.

Introduction: Due to a lack of robust population denominators, Australia is unable to accurately monitor changes in the HIV epidemic for some populations. The ratio of HIV transmission relative to the number of people with HIV (an incidence prevalence ratio, or IPR) can measure such changes. The IPR is measured against an IPR benchmark derived from post-HIV acquisition life expectancy, to indicate whether an HIV epidemic is shrinking or growing. Using IPRs and Australia-specific IPR benchmarks, this study aims to describe the Australian HIV epidemic among three groups: men with HIV attributed to male-to-male sex, women with HIV and people with HIV attributed to injection drug use.

Methods: Using mathematical modelling derived from HIV notifications, cohort and administrative data, IPRs were generated for each of the three groups. These IPRs were compared with IPR benchmarks derived from post-HIV acquisition mortality estimates using abridged life tables for men, women and people who inject drugs. The IPR benchmark for men was applied to people with HIV attributed to male-to-male sex. Trends in the IPR over time were described for each reported population from 2015 to 2022.

Results: Overall, the IPR fell by 80%, from 0.040 (range: 0.034-0.045) in 2015 to 0.008 (range: 0.003-0.013) in 2022 and fell below the benchmark (0.022) in 2020. Among people with HIV attributed to male-to-male sex, the IPR fell by 85%, from 0.041 (range: 0.034-0.047) in 2015 to 0.006 (range: 0.003-0.024) in 2022 and fell below the benchmark (0.022) in 2020. Among women with HIV, the IPR fell by 56%, from 0.032 (range: 0.026-0.039) in 2015 to 0.014 (range: 0.003-0.029) in 2022 and fell below the benchmark (0.022) in 2019. Among people with HIV attributed to injection drug use, the IPR fell by 61%, from 0.036 (range: 0.022-0.047) in 2015 to 0.014 (range: 0.002-0.057) in 2022 and fell below the benchmark (0.028) in 2019.

Conclusions: Australian IPRs in all populations examined have dropped below the level required to sustain the HIV epidemic at current levels. By applying this method in other contexts, the changing scale of HIV epidemics may be better described for populations lacking robust population denominators.