Background Cohort studies on hepatitis C virus (HCV) among drug injectors are scarcer than studies on HIV. Combined harm reduction interventions (HRIs) can prevent HCV infection. Spain has a medium–high coverage of HRIs.

Methods 513 young heroin users who injected drugs in the past 12 months (recent injectors) were street-recruited in 2001–2003 and followed until 2006 in three Spanish cities; 137 were anti-HCV seronegative, 77 of whom had ≥1 follow-up visit. Dried blood spots were tested for anti-HCV. HCV incidence and predictors of infection were estimated using Poisson models.

Results At baseline, 73% were anti-HCV positive. Overall incidence (n=77) of HCV seroconversion was 39.8/100 person-years (py) (95% CI 28.7 to 53.8). Excluding non-injectors during follow-up from the analysis (n=57), HCV incidence was 52.9/100 py (95% CI 37.4 to 72.5). Injecting at least weekly (incidence rate ratio (IRR)=5.2 (95% CI 2.5 to 11.1)) and having ≥2 sexual partners (IRR=2.2 (95% CI 1.1 to 4.7)) were independent predictors of HCV seroconversion; drug-injection history <2 years was marginally associated (IRR=2.4 (95% CI 0.9 to 4.7)). HCV incidence may have been underestimated due to differential attrition.

Conclusions Despite fairly high HRI coverage among Spanish drug injectors, a distressingly high incidence of HCV in a context of high HCV prevalence was found among young heroin injectors.