The explosive and ongoing HIV-1 epidemic related to injecting drug use in Vancouver continues to receive international attention. This has been particularly perplexing due to the presence of legal needle exchange programs (NEP), access to methadone treatment, and a range of social programs. Compelling evidence supports the role of these initiatives in reducing HIV transmission; yet the debate around the efficacy of NEPs has been fueled by Vancouver's HIV epidemic. The choice of drugs and the patterns of their use may help explain the wide variability in HIV incidence and prevalence among different drug-using populations.

In Vancouver, the use of multiple injected and non-injected drugs is prevalent, with cocaine being the predominant injection drug used in the community. The present study was designed to identify factors associated with HIV infection in this community and to determine how drug use patterns influence HIV risk over time. The researchers also evaluated whether a dose-dependent relationship exists with regard to HIV seroconversion and the frequency of injecting cocaine and heroin.

The Vancouver Injection Drug Users Study is an open prospective cohort of injecting drug users that began in May 1996. At enrollment and at semi-annual follow-up visits, an interviewer administers a detailed semi-structured questionnaire. Cox proportional-hazards models were used to determine behavioral and drug use patterns reported in the six months prior to HIV seroconversion.

From 940 seronegative VIDUS participants, 109 incident HIV infections were observed during a mean follow-up of 31 months. During the six months prior to conversion, predictors of HIV infection were injecting cocaine use (adjusted hazards ratio 3.72), incarceration (AHR 2.74), unstable housing (AHR 2.36), methadone maintenance treatment (MMT) (AHR 1.98), and Aboriginal ethnicity (AHR 1.78). Injecting cocaine use was predictive of HIV infection in a dose-dependent fashion. Compared with infrequent cocaine users, those who average more than three injections daily were seven times more likely to contract HIV. The time to HIV infection was accelerated among regular cocaine injectors independent of concurrent heroin use.

"Participants reporting multiple injections of cocaine over short periods of time are common, in contrast to those individuals who consistently use heroin and generally require two to four daily injections to avoid withdrawal symptoms. The consistent daily use of heroin lends itself far more than cocaine to regular injection practices, including a more predictable source of drugs and a more orderly preparation of injection paraphernalia. These observations may help to explain the very strong dose-response for HIV seroconversion among cocaine users and the lack of this association for heroin users," the authors wrote. "We have recently found that difficulty accessing sterile syringes is the primary risk factor for syringe sharing in Vancouver, and this observation, combined with the role of cocaine injection identified in the present study, may help to explain why an HIV epidemic has been observed in Vancouver despite the NEP."

The authors noted that the use of crack cocaine in the previous six months was inversely related to HIV seroconversion; this may be explained by the substitution of inhaled crack cocaine by some IDUs who wish to avoid needles. They also noted it is "possible that some individuals taking MMT may indeed be at higher risk of HIV infection due to increased injection of cocaine."

In conclusion, the authors wrote, "Interventions currently available for cocaine dependency are woefully inadequate. The development of both pharmacologic and behavioral interventions must be a high priority. ... Success in reducing the incidence of HIV infection among injecting drug users will only be realized by providing relevant interventions in a timely fashion. The catastrophic outbreak of HIV infection in Vancouver during the mid-1990s cannot be attributed to any single event or environmental factor. However, cocaine use in this population has clearly had a major influence on HIV transmission and has contributed in a large part to the epidemic. ..."

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