The causal effect of opioid agonist treatment on adherence to direct-acting antiviral treatment for hepatitis C virus

Abstract

Background. Opioid agonist treatment (OAT) supports adherence in medication regimens for other concurrent conditions. However, sparse evidence is available on its effect on promoting retention to direct-acting antivirals (DAAs) for people with opioid use disorder (PWOUD) with concurrent hepatitis C virus (HCV). Our objective was to determine the causal impact of OAT exposure on DAA adherence among HCV-positive PWOUD. Methods. We executed a retrospective study using linked population-level data for British Columbia, Canada (January 1996–September 2018). We estimated the effect of OAT on DAA adherence using generalized estimating equations (GEEs) and marginal structural modeling (MSM) for time-varying confounding. The primary outcome was 85% DAA adherence (minimum 6 of 7 days). Results. We included 2820 HCV-positive PWOUD who initiated a DAA regimen (32.6% female, 83.9% previously accessing OAT), with 2410 (95% among uncensored episodes) completing the regimen. The GEE-adjusted odds ratio of DAA adherence after OAT exposure was 1.05 (0.89–1.23), whereas the MSM-adjusted odds ratio was 0.97 (0.78–1.22). Conclusions. In a setting with universal healthcare and widespread access to OAT and DAA treatment, DAA regimen completion rates were high regardless of OAT, and engagement in OAT did not increase DAA adherence. Nonengagement in OAT should not preclude DAA treatment for PWOUD.