Background & Aims
Understanding HCV transmission among people who inject drugs (PWID)
is important for designing prevention strategies. This study investigated
whether HCV infection among younger injectors occurs from few or many
transmission events from older injectors to younger injectors among PWID in
Vancouver, Canada.
Methods
HCV antibody-positive participants at enrolment or follow-up
(1996-2012) were tested for HCV RNA and sequenced (Core-E2). Time-stamped
phylogenetic trees were inferred using Bayesian Evolutionary Analysis
Sampling Trees (BEAST). Association of age with phylogeny was tested using
statistics implemented in the software Bayesian Tip Significance (BaTS)
testing. Factors associated with clustering (maximum cluster age: five
years) were identified using logistic regression.
Results
Among 699 participants with HCV subtype 1a, 1b, 2b and 3a infection
(26% female, 24% HIV+): 21% were younger (<27 years), and 10% had
recent HCV seroconversion. When inferred cluster age was limited to
<5 years, 15% (n=108) were in clusters/pairs. Although a moderate
degree of segregation was observed between younger and older participants,
there was also transmission between age groups. Younger age (<27 vs.
>40, AOR: 3.14, 95% CI:1.54, 6.39),HIV (AOR: 1.97, 95%-CI: 1.22,
3.18) and subtype 3a (AOR: 2.12, 95% CI: 1.33, 3.38) were independently
associated with clustering.
Conclusions
In this population of PWID from Vancouver, HCV among young injectors
was seeded from many transmission events between HCV-infected older and
younger injectors. Phylogenetic clustering was associated with younger age
and HIV. These data suggest that HCV transmission among PWID is complex,
with transmission occurring between and among older and younger PWID.