In 2012, less than 1% of the estimated 12,000 hepatitis C (HCV) infected people who inject drugs (PWID) in British Columbia (BC) received HCV treatment, despite accounting for 80% of all new HCV infections. The recent approval and availability of high-efficacy and tolerable HCV medications will make it possible to treat a large number of individuals who were previously ineligible for treatment. Reinfection risk remains an issue, particularly among PWID, and it is still not clear if individuals who achieve sustained virologic response gain some degree of subsequent immunity. Reinfection risk could also be mitigated by engaging individuals in harm reduction programs.
We designed a deterministic compartmental mathematical model of HCV disease transmission fit to the PWID population in BC, based on treating a fixed number of individuals per year. We calculated the difference in incident cases with respect to the status-quo, as a function of both the number of PWID treated per year, and varying rates of reinfection risk. We defined the threshold for HCV control as the minimum number of PWID treated per year required to offset the number of new incident cases, with removals taken into account.
The control threshold at year five (Figure 1A) ranged from 128 PWID treated per year, assuming 0% reinfection risk, to 178 when there is 100% reinfection risk, i.e., equal to naïve uninfected PWID. At ten years (Figure 1B), the threshold varied between 121 and 240 individuals treated. We simulated the change in incident cases (Figures 1C-1D) when treating 100 or 300 PWID per year, for varying rates of reinfection risk reduction. In the first scenario (100 treated per year, Figure 1C), after 20 years of constant treatment uptake, the number of incident cases was reduced between 1% and 16%. In the second scenario (300 treated per year, Figure 1D), the number of incident cases decreased between 16% and 54%.
The availability of highly efficacious treatments holds great promise to disrupt the course of the HCV epidemic. Treating the PWID population is crucial to controlling the epidemic, as the majority of new infections occur within this population, but reinfection risk remains a concern. Our simulations show that treating a minimum of 200 to 300 PWID per year will lead to HCV control, and this will be substantially accelerated if the potential for HCV reinfection is minimized through the deployment of harm reduction programs.