Modelling the impact of incarceration and prison-based HCV treatment on HCV transmission among people who inject drugs in Scotland

Background/Aims: People who inject drugs (PWID) experience high incarceration rates, and previous incarceration is associated with elevated hepatitis C virus (HCV) transmission risk. We evaluate the contribution of incarceration to HCV transmission amongst PWID, and the impact of prison-related prevention interventions, including scaling-up direct-acting antivirals (DAAs) in prison. 
Design: Dynamic mathematical modelling of incarceration and HCV transmission, using approximate Bayesian computation for model calibration.
Setting: Scotland; where national survey data indicates lower HCV incidence in prison than the community (4.3 vs 7.3 per 100py), but a 2.3-fold elevated transmission risk amongst recently released (<6 months) PWID.
Participants: PWID
Measurements: Population attributable fraction of incarceration (PAF) to HCV transmission among PWID. Decrease in HCV incidence and chronic prevalence due to current levels of prison opiate substitution therapy (OST; 57% coverage) and HCV treatment; as well as scaling-up DAAs in prison and/or preventing the elevated risk associated with prison-release.
Findings: Incarceration contributes 27.7% (PAF; 95%CrI -3.1-51.1%) of HCV transmission amongst PWID in Scotland. Over the next 15 years, current HCV treatment rates (10.4/6.8 per 1000 incarcerated/community PWID annually), with existing prison OST, could reduce incidence and chronic prevalence among all PWID by a relative 10.7% (95%CrI 8.4-13.3%) and 9.7% (95%CrI 7.7-12.1%), respectively. Conversely, without prison OST, HCV incidence and chronic prevalence would decrease by 3.1% (95%CrI -28.5-18.0%) and 4.7% (95%CrI -11.3-14.5%). Additionally, preventing the heightened risk among recently released PWID could reduce incidence and chronic prevalence by 45.0% (95%CrI 19.7-57.5%) and 33.3% (95%CrI 15.6-43.6%) or scaling-up prison HCV treatments to 80% of chronic PWID prison entrants with sufficient sentences (>16 weeks) could reduce incidence and prevalence by 45.6% (95%CrI 38.0-51.3%) and 45.5% (95%CrI 39.3-51.0%), respectively.
Conclusions: Incarceration and the elevated transmission risk following prison-release can contribute significantly to HCV transmission amongst PWID. Scaling-up HCV treatment in prison can provide important prevention benefits.