Injecting frequency trajectories and hepatitis C virus acquisition: Findings from a cohort of people who inject drugs in Montréal, Canada

Background
Frequent injecting increases hepatitis C (HCV) acquisition risk among people who inject drugs (PWID). However, few studies have examined how temporal fluctuations in injecting frequency may effect HCV infection risk. Thus, this study examined HCV incidence according to injecting frequency trajectories followed by PWID over one year in Montréal, Canada.

Methods
At three-month intervals from March 2011 to June 2016, HCV-uninfected PWID (never infected or cleared infection) enrolled in the Hepatitis Cohort (HEPCO) completed interviewer-administered questionnaires and HCV testing. At each visit, participants reported the number of injecting days (0–30 days) for each of the past three months. In previous work, using group-based trajectory modeling, we identified five injecting frequency trajectories followed by participants over one year (months 1–12 of follow-up), including sporadic, infrequent, increasing, decreasing, and frequent injecting. In this study, we estimated group-specific HCV incidence (months 1–63 of follow-up) using posterior probabilities to assign participants to their most likely trajectory group.

Results
Of 386 participants (mean age=40, 82% male, 48% never HCV-infected), 72 acquired HCV during 893 person-years of follow-up. HCV incidence for the whole study sample was 8.1 per 100 person-years (95%CI=6.4–10.1). Trajectory group-specific HCV incidences were highest for those injecting drugs with decreasing (23.9, 14.4–37.5) or increasing frequency (16.0, 10.1–24.3), intermediate for those injecting at consistently high frequency (10.2, 5.4–17.8), and lowest for those injecting infrequently (3.9, 2.2–6.5) or sporadically (4.3, 2.2–7.6).

Conclusion
Results suggest that PWID at highest HCV risk are those injecting at high frequency, either transitorily (increasing, decreasing injecting) or consistently (frequent injecting). This study highlights changes in injecting frequency as a potentially important dimension to consider among the factors leading to HCV acquisition. Clinical and public health interventions tailored to PWID with different injecting frequency profiles may contribute to HCV prevention.