Modelling the impact of a national scale-up of interventions on hepatitis C virus transmission among people who inject drugs in Scotland

Background and Aims: To reduce hepatitis C virus (HCV) transmission among people who inject drugs (PWID), Scottish Government-funded national strategies, launched in 2008, promoted scaling-up opioid substitution therapy (OST) and needle and syringe provision (NSP), with some increases in HCV treatment. We test whether observed decreases in HCV incidence post-2008 can be attributed to this intervention scale-up.
Design: A dynamic HCV transmission model amongst PWID incorporating intervention scale-up and observed decreases in behavioural risk, calibrated to Scottish HCV prevalence and incidence data for 2008/09.
Setting: Scotland, UK
Participants: PWID
Measurements: Model projections from 2008-2015 were compared with data to test whether they were consistent with observed decreases in HCV incidence amongst PWID while incorporating the observed intervention scale-up, and to determine the impact of scaling-up interventions on incidence.
Findings: Without fitting to epidemiological data post-2008/09, the model incorporating observed intervention scale-up agreed with observed decreases in HCV incidence amongst PWID between 2008-2015, suggesting HCV incidence decreased by 61.3% (95% credibility interval 45.1-75.3%) from 14.2/100pyrs (9.0-20.7) to 5.5/100pyrs (2.9-9.2). On average, each model fit lay within 84% (10.1/12) of the confidence bounds for the 12 incidence data points which the model was compared against. We estimate that scale-up of interventions (OST+NSP+HCV treatment) and decreases in high-risk behaviour from 2008-2015 resulted in a 33.9% (23.8-44.6%) decrease in incidence, with the remainder (27.4% (17.6-37.0%)) explained by historical changes in OST+NSP coverage and risk pre-2008. Projections suggest scaling-up of all interventions post-2008 averted 1,492 (657-2,646) infections over 7-years, with 1,016 (308-1,996), 404 (150-836) and 72 (27-137) due to scale-up of OST+NSP, decreases in high-risk behaviour, and HCV treatment, respectively.
Conclusions: Most of the decline in hepatitis C virus (HCV) incidence in Scotland between 2008-2015 appears to be attributable to intervention scale-up (opioid substitution therapy and needle and syringe provision) due to government strategies on HCV and drugs.