Social structural and behavioral underpinnings of hyperendemic hepatitis C virus transmission in drug injectors

Background. Hepatitis C virus (HCV) is hyperendemic in drug injectors, yet social structural and behavioral factors underlying transmission are not well established. Methods. We conducted a case-control study of HCV seroconversion in drug injectors, focusing on transmission within networks. Incident case subjects (n = 17) and seronegative control subjects (n = 42) reported injection and sex partners and referred as many as 5 for interviewing and blood testing. We performed nucleotide sequencing of HCV isolates from infected individuals. Results. Seventy-eight percent of recent injection partnerships involved behavior that could transmit HCV. Case subjects and control subjects were similar demographically and behaviorally. Case subjects, however, had more HCV-infected partners and consequently engaged in injection risk behavior with more infected partners. The injection network was mostly connected, dense, and cyclic, but the sexual network was highly fragmented. Although participants generally injected with partners of similar age, most HCV-uninfected participants recently had injected with infected partners. In at least 1 of 4 pairs of genetically linked infections, transmission appeared to be due to sharing of injection equipment other than syringes. Except for transmission pairs, network distance between incident case subjects and genetic distance between their HCV variants were uncorrelated. Conclusions. Without dramatic reductions in injection risk behaviors, shattering of cohesive injection networks, and/or broad coverage of an effective vaccine, HCV will likely remain hyperendemic in drug injectors.