Hepatitis C virus infection among injection drug users: survival analysis of time to seroconversion

Time to hepatitis C virus (HCV) seroconversion in initially seronegative injection drug users has not been directly measured, and public health planning would benefit from specifying the window of opportunity for prevention of infection, and factors that affect timing of infection.

Four hundred eighty-four HCV antibody-negative injection drug users in Seattle, Washington were followed a median of 2.1 years to observe seroconversion. We examined time to HCV seroconversion in relation to subject characteristics using the Kaplan-Meier method and Cox proportional hazards regression. A weighted-average time to HCV seroconversion was calculated among new injectors (injecting < or = 2 years) using seroprevalence and seroincidence data.

There were 134 HCV seroconversions (11.6 per 100 person-years at risk; the 25th percentile of time to seroconversion was 26.2 months). Injection with a syringe used by another injector (adjusted hazards ratio = 1.8; 95% confidence interval = 1.3-3.0) and sharing a cooker or cotton (1.8; 1.0-3.1) were associated with time to HCV seroconversion. Using the estimate of the mean time to seroconversion from first injection in new injectors who were HCV antibody-negative at enrollment (5.4 years), and the midpoint between first injection and study enrollment in new injectors who were HCV antibody-positive at enrollment (0.6 years), the weighted-average time to seroconversion after beginning to inject was estimated to be 3.4 years.

The period of susceptibility to HCV infection in the majority of drug injectors appears to be long enough to justify the allocation of substantial resources toward interventions to reduce injection-related risk behavior in these individuals.