| Summary: | <h4>Background</h4>Shared injecting apparatus during drug use is the premier risk factor for hepatitis C virus (HCV) transmission.<h4>Aims</h4>To estimate the per-event probability of HCV infection during a sharing event, and the transmission probability of HCV from contaminated injecting apparatus.<h4>Methods</h4>Estimates were obtained using a maximum likelihood method with estimated IDU and sharing events obtained from behavioural data.<h4>Settings</h4>Cohort study in multiple correction centres in New South Wales, Australia.<h4>Participants</h4>Subjects (N = 500) with a lifetime history of injecting drug use (IDU) who were followed up between 2005 and 2012. During follow-up, interviews for risk behaviours were taken and blood sampling (HCV-antibody and RNA testing) was performed.<h4>Measurements</h4>Self-reported frequencies of injecting drugs and sharing events, as well as other risk behaviours and details on the nature of injecting events.<h4>Findings</h4>The best estimate of the per-event probability of infection was 0.57% (CI: 0.32-1.05%). A sensitivity analysis on the likely effect of under-reporting of sharing of the injecting apparatus indicated that the per event infection probability may be as low as 0.17% (95% CI: 0.11%-0.25%). The transmission probability was similarly shown to range up to 6%, dependent on the presumed prevalence of the virus in injecting equipment.<h4>Conclusions</h4>The transmission probability of HCV during a sharing event is small. Hence, strategies to reduce the frequency and sharing of injecting equipment are required, as well as interventions focused on decreasing the per event risk.
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