We documented a sustained high incidence of HCV among IDUs and found that seroincident infection was independently associated with female sex, cocaine use, frequent injection and frequent attendance at a needle exchange program.
The relation between HIV and HCV seropositivity among IDUs is not always consistent, but it has been hypothesized that positive associations are likely where the prevalence of both pathogens is high.25
This dual high-prevalence condition is satisfied in the VIDUS cohort, in which there was a clear association between prevalences of HIV and HCV infection at baseline. However, the study documents interesting comparative trends in the rates of HIV and HCV seroincidence among IDUs. Although HIV incidence was high at study inception, it subsequently declined, as had been predicted through mathematical modelling.26
In contrast, despite a prevalence of 81.6% at baseline, HCV incidence remained above 16 per 100 person-years over the 3-year period of observation. High rates of incidence for HCV are well documented among young and new IDUs,15,27
such that 80% become infected within 4 years of initiating injection drug use.28
High rates of HCV transmission are explained by a combination of high risk of infection for each syringe- sharing contact and high infectivity throughout much of the natural history of an infection (and consequent high prevalence among syringe lenders), as well as the frequency of syringe-sharing contacts in a given community.29
The infection rate following needlestick exposure to HCV appears to be at least an order of magnitude greater than that following similar exposure to HIV.30
Factors associated with HCV seroconversion in this study resemble those previously shown to predict HIV infection in the same cohort2
and include frequent injection and use of cocaine either alone or combined with heroin as a speedball. Higher frequency of injection and more chaotic drug use have been associated with cocaine use in a number of cohort studies. Although borrowing used syringes is the most likely operative mechanism of infection and a widely reported predictor of HCV infection among IDUs,13
socially desirable responses may explain why it was not a more significant direct measure of risk in this study. Measures of high frequency of injection may be a better, albeit surrogate, measure of this core risk behaviour. Conversely, other researchers have reported that practices such as sharing other injection paraphernalia (e.g., cookers, cotton or water) and “front-end loading” (syringe-mediated drug sharing) may contribute to transmission above and beyond that associated with simple syringe sharing.14,20,31
This finding suggests that prevention messages aimed at IDUs must go beyond simply advising against sharing needles and should discourage sharing of any injection equipment.
The association of HCV seroincidence with female sex suggests a higher vulnerability to parenteral exposure, an important role for sexual exposure or both. Romanowski and associates13
also reported associations between high prevalence of HCV infection and the sex trade, but these associations did not remain significant on multivariate analysis. Sexual transmission of HCV has been documented but appears relatively inefficient.32,33,34
In studies of HCV transmission, associations with numbers of partners, condom use and commercial sex have been less consistently demonstrated in populations of IDUs than in other affected populations.8,35
We and others have previously reported an association between HIV infection and frequent attendance at needle exchange programs.2,21,36
We now report a similar association for HCV infection, an association that may result from unmeasured confounders — factors promoting transmission that are common among those frequently attending needle exchange programs. However, such findings have also been interpreted as suggesting that needle exchange programs may actually promote HIV transmission.37,38
It should be noted that a close analysis of the association between HIV and frequency of attendance at needle exchange programs for the Vancouver IDU cohort did not support a hypothesis of causality.3
High and increasing rates of HCV infection have been documented in Canadian prison populations.39
Our study illustrates that IDUs who have been incarcerated are especially likely to be infected with HCV.
This study had several limitations. Because of high HCV prevalence at enrolment and limited follow-up to date, the statistical power to determine incidence and associated risk factors was low. Participants who were initially HCV seronegative returned for follow-up less frequently than did study subjects in general. It is plausible that some HCV-negative IDUs were not highly street entrenched (i.e., were not involved in a chronic and intractable way with the street scene) and were therefore less likely to return for a follow-up visit. Indeed, some of the participants who were negative for HCV at baseline were deported from Canada during the follow-up period. Also, the study instrument did not address the practices of tattooing and body piercing, which have been associated elsewhere with HCV infection.35,40,41,42
Observations in this study were made over the course of a documented outbreak of HIV. This and the unique characteristics of the Vancouver injection drug use scene may limit the generalizability of the results.
HCV infection represents a major cause of morbidity and stands as a sensitive marker for activities that may transmit HIV parenterally. Because this and other studies indicate extreme pressure toward HCV transmission among seronegative IDUs, resulting from both high prevalence and high infectivity, it appears that the prevention efforts must be expanded yet further.
Mathematical models suggest that even in populations with a high prevalence of HCV infection, there is hope that a drop in syringe sharing will reduce the incidence of such infection.43
However, such reductions have been hard to realize. Because of high transmissibility of HCV among those injecting frequently and using cocaine, we conclude that the harm reduction initiatives deployed in Vancouver during the study period proved insufficient to eliminate hepatitis C transmission in this population. In addition to promoting behavioural change and harm reduction among established users, researchers and prevention workers urgently need to focus on primary prevention of injection drug use, early intervention with and treatment of non-injection and injection drug users, and altering the social determinants that predispose people to engage in these activities.