Objective. To examine hepatitis C virus (HCV) seroprevalence among injection drug users in 4 US cities from 1994 through 2004. Methods. Demographic characteristics, behaviors, and prevalence of HCV antibody among 5088 injection drug users aged 18-40 years from Baltimore, Maryland; Chicago, Illinois; Los Angeles, California; and New York, New York, enrolled in 3 related studies - Collaborative Injection Drug User Study (CIDUS) I (1994-1996), CIDUS II (1997-1999), and CIDUS III/Drug User Intervention Trial (2002-2004) - were compared using the χ2 and Mantel-Haenszel tests of significance. Trends over time were assessed by logistic regression. Results. Prevalence of HCV infection was 65%, 35%, and 35% in CIDUS I, CIDUS II, and CIDUS III, respectively. The adjusted prevalence odds ratio (OR) of being HCV antibody positive increased with the number of years of injection drug use (OR, 1.93 [95% confidence interval {CI}, 1.68-2.21] for each year of injecting within the first 2 years; OR, 1.09 [95% CI, 1.07-1.11] for each year of injecting beyond the first 2 years). Significant decreases were observed in the prevalence of HCV antibody between CIDUS I and CIDUS III in Baltimore (OR, 0.30; 95% CI, 0.20-0.43) and Los Angeles (OR, 0.17; 95% CI, 0.09-0.31) and among people of races other than black in Chicago (OR, 0.12; 95% CI, 0.08-0.17). No decrease in prevalence was seen in New York (OR, 1.04; 95% CI, 0.69-1.58) or among blacks in Chicago (OR, 0.55; 95% CI, 0.16-1.90). Conclusion. Although regional differences exist, our data suggest that the incidence of HCV infection among injection drug users in the United States decreased from 1994 through 2004. © 2008 by the Infectious Diseases Society of America. All rights reserved.
