Per-Event Probability of Hepatitis C Infection during Sharing of Injecting Equipment

<div><p>Background</p><p>Shared injecting apparatus during drug use is the premier risk factor for hepatitis C virus (HCV) transmission.</p><p>Aims</p><p>To estimate the per-event probability of HCV infection during a sharing event, and the transmission probability of HCV from contaminated injecting apparatus.</p><p>Methods</p><p>Estimates were obtained using a maximum likelihood method with estimated IDU and sharing events obtained from behavioural data.</p><p>Settings</p><p>Cohort study in multiple correction centres in New South Wales, Australia</p><p>Participants</p><p>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.</p><p>Measurements</p><p>Self-reported frequencies of injecting drugs and sharing events, as well as other risk behaviours and details on the nature of injecting events.</p><p>Findings</p><p>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.</p><p>Conclusions</p><p>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.</p></div

Number of interviews with self-reported injecting and sharing behaviours in the prospective (A) and retrospective (B) cohorts.

<p>Number of interviews with self-reported injecting and sharing behaviours in the prospective (A) and retrospective (B) cohorts.</p

A. Sensitivity analysis to assess the effect of under-reporting of sharing during IDU.

<p>Modelling of the effect of under-reporting of sharing during IDU on the estimate of the per-event probability of infection during IDU with shared equipment. On the x-axis, the proportion <i>s</i> of IDU events that are also sharing events is represented. Boxplots represent the simulation-based distribution of the MLE for the estimate (for comparison, the actual result from the reported sharing distribution in the cohort is also reported). B. Boxplots representing the distribution of the MLE for the transmission probability in the population, as a function of the assumed proportion injecting equipment actually contaminated with HCV.</p

Demographic characteristics and injecting behaviours in the two study cohorts.

1<p>Estimate based on intermediate estimate from categorical data available on IDU frequency, for subjects who reported sharing.</p

Maximum likelihood estimate of the per-event probability of HCV infection during IDU events with sharing of injecting apparatus.

<p>* Estimated from bootstrap analysis using 10,000 simulations (see Materials & Methods).</p

Numerical quantification (in times per week) of the injecting and sharing events.

<p>Panel A. Numerical quantification (in times per week) of the injecting (both cohorts), and sharing events from categorical answers of the prospective cohort, provided by subjects during the HITS-p interviews to the questions “Since the last interview, how often did you inject drugs?” and “Since the last interview, how often did you use injecting equipment after someone else had used it?”. Panel B. Numerical quantification (as a percentage of the number of IDU events) of the sharing events in the retrospective cohort, based on the question “Since last imprisonment, how often did you use equipment after someone else had used it?”.</p