Frequent hepatitis C virus superinfection in injection drug users.

The frequency of hepatitis C virus (HCV) superinfection with a divergent viral strain was determined in a cohort of recently infected young injection drug users (IDUs) with an HCV incidence rate of 25%. HCV was amplified, by use of polymerase chain reaction (PCR), from plasma samples collected from 25 HCV-infected individuals over an average period of 12 months, and their viral sequences were compared. Phylogenetic analysis identified 5 IDUs with superinfection (20%) occurring after seroconversion: 2 IDUs were superinfected with different HCV genotypes, and 3 were superinfected with divergent strains of the same genotype. The superinfecting strains were not detected as minority variants (<0.5%) in the initial plasma HCV quasi species. Extensive measures were taken to exclude PCR contamination and mix-up of samples, and superinfection results were concordant at 2 HCV genetic loci. HCV superinfection in IDUs, both intra- and intergenotype, is therefore a frequent event, with an incidence rate similar to that of de novo infections. These results suggest that no cross-protecting immunity develops during the first year of chronic infection with HCV.