Modeling has suggested HCV prevalence reductions of >50% with widespread treatment in people who inject drugs [PWID]). However, there has been inadequate consideration of 1) how much transmission can be targeted by treatment given realistic delays and empirical data on age specific hazards; and 2) population-level mixing characterizing counterfactual transmission if those treated remain untreated and continue to infect. We explore the impact of treatment strategies on HCV incidence using data from multi-decade studies of HCV transmission among PWID in Baltimore, Maryland.
We developed an age-specific compartmental model of HCV transmission in a community of current and former PWID parameterized using empirical data from studies in Baltimore to obtain age-specific 1) HCV prevalence from 1988-2008; 2) rates of injection initiation, cessation and relapse; and 3) mortality. We varied contact matrices from random to fully age-specific. We compared strategies from conservative: 1) treating only abstinent PWID ~ 15 years after infection to aggressive: 2) treating all PWID regardless of injection 1-5 years after infection with and without harm reduction scale-up. We estimated reductions in incidence/prevalence over 20 years.
Our model supports that widespread HCV treatment can have significant positive impact – reduction in prevalence of 40% with 20,000 treatment courses over 20 years. At this level, HCV prevalence decrease varied little by who was treated (active, abstinent vs. all) or when they received it (1-15 years after infection, Figure). Further, at coverage <88% of the PWID population, almost all prevalence reduction was due to direct effects of curing people. Indirect effects were negligible because the hazard of HCV infection is so high that significant treatment is needed to reduce it. In order to impact transmission (indirect effects), treatment needs to be scaled to >90% of the population (>40,000 doses) targeted 1-3 years after infection with simultaneous harm reduction scale up. Even at these levels, only 0.8 incident cases are averted per treatment.
To truly impact HCV transmission in PWID, treatment programs need to be aggressive in treating large numbers of PWID almost immediately after HCV acquisition and comprehensive by integrating harm reduction. Given the vast amount of treatment need to impact transmission, programs should prioritize clinical considerations and the relative impact of harm reduction.