Objectives: We aim to estimate how changes in sterile syringe distribution through needle-syringe programs (NSPs) may affect HIV and hepatitis C virus (HCV) incidence among injecting drug users (IDUs) in Australia.
Methods: We develop a novel mathematical model of HIV and HCV transmission among IDUs who share syringes. It is calibrated using biological and Australian epidemiological and behavioral data. Assuming NSP syringe distribution affects the number of times each syringe is used before disposal, we use the model to estimate the relationship between incidence and syringe distribution.
Results: HIV is effectively controlled through NSP distribution of sterile syringes {with the effective reproduction ratio below 1 [0.66 median, interquartile range (0.63-0.70)] under current syringe distribution}. In contrast, HCV incidence is expected to remain high and its control is not feasible in the foreseeable future. The proportion of injections that are shared and the number of times each syringe is used before disposal are the driving factors of HCV incidence. The frequency in which each syringe is used can potentially be influenced by changes in syringe distribution. We estimate that if syringe distribution or coverage doubled, then annual incidence is likely to reduce by 50%. However, if it was decreased to one third of the current level, then ∼3 times the incidence could be expected.
Conclusions: This research highlights the large benefits of NSPs, puts forward a quantitative relationship between incidence and syringe distribution, and indicates that increased coverage could result in significant reductions in viral transmissions among IDUs.