Estimating population attributable risk for hepatitis C seroconversion in injecting drug users in Australia:...

To determine risk factors and estimate their population-level contribution to hepatitis C virus (HCV) burden.

Established and potentially modifiable risk factors were estimated using partial population attributable risk (PAR(p)) in a cohort of new injecting drug users (IDUs) in Sydney, Australia.

A total of 204 hepatitis C seronegative IDUs were recruited through street-based outreach, methadone clinics and needle and syringe programmes (NSPs) and followed-up at 3-6-monthly intervals. A total of 61 HCV seroconversions were observed during the follow-up [overall incidence rate of 45.8 per 100 person-years (95% confidence interval: 35.6-58.8)]. Overall, five potentially modifiable risk factors (sharing needles/syringes, sharing other injecting equipment, assisted injecting, frequency of injection and not being in drug treatment) accounted for approximately 50% of HCV cases observed.

While sharing needles/syringes or other injecting equipment were associated most strongly with increased risk of HCV infection, the PAR(p) associated with these behaviours was relatively modest (12%) because they are relatively low-prevalence behaviours. Our analyses suggest that more HCV infection could be avoided by changing more common, but less strongly associated behaviours such as assisted injecting or daily injecting. Results suggest that to have a very substantial effect on HCV, a range of risk factors need modifying. The most efficient use of scarce resources in reducing HCV infections will require complex balancing between the PAR for a given risk factor(s), the efficacy of interventions to actually modify the risk factor, and the cost of these interventions.