Why the large difference in hepatitis C virus (HCV) infection prevalence between young injecting drug users (IDUs) in Baltimore and Chicago?

Objective: Delineate factors associated with HCV prevalence among non-Hispanic (NH) white IDUs from Chicago (n=586) and Baltimore (n=736). Methods: IDUs 15-30 years old participated in the Third Collaborative Injection Drug Users Study/Drug User Intervention Trial through street outreach and respondent-driven recruiting. Baseline computerized, self-administered interviews and serological data were evaluated. Results: A striking difference in HCV prevalence exists between NH-white Baltimore and Chicago IDUs (53% vs. 14%, p<0.0001). In comparison to Chicago, Baltimore HCV-infected+ IDUs were significantly (p<0.05) more likely to be male, older, primarily inject cocaine, inject drugs >=5 times/day, inject with a used syringe, trade sex for drugs/money, ever have had hepatitis A or B, and not utilize a syringe exchange programs. Similar proportions of HCV+ IDUs from both cities were HIV+, injected drugs >=2 years, and shared injection paraphernalia. In multivariable logistic regression, most of these factors remained moderately significant (p<0.05) with odds ratios (OR) between 1.0 and 3.4; however, Baltimore city was the strongest correlate of HCV prevalence (OR=5.6, 95% confidence interval: 3.5-5.7). Geospatial analysis revealed that HCV infection in Baltimore clustered in five 5 adjacent zip codes where NH-Whites were 72%-95% of the population, but no such clustering of HCV was found among Chicago IDUs. Conclusion: Differences in injection risk behaviors only partially explain the difference in HCV prevalence between young IDUs in Baltimore and Chicago. HCV clustering in Baltimore but not Chicago suggests that social network and neighborhood characteristics are also key determinants of HCV acquisition and should be examined in future research.