Estimating incidence rates of primary infection and reinfection with hepatitis C virus among people who inject drugs in Scotland: a model-based analysis of repeated cross-sectional survey data

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Background: Reducing the incidence of hepatitis C virus (HCV) infection is a World Health Organization (WHO) elimination goal, but approaches to estimate this from population-level survey data are lacking. We modelled HCV incidence among people who inject drugs (PWID) surveyed over time, to provide evidence on whether Scotland has reached the WHO elimination target of ≤2 per 100 person-years in this population. Methods: A statistical model was fitted using HCV infection data from five sweeps (2013-14, 2015-16, 2017-18, 2019-20, and 2022-23) of a national bio-behavioural survey, involving 11,651 PWID recruited at sites providing injecting equipment. Per-sweep incidence rates of primary chronic HCV infection, reinfection, and combined primary and reinfection ('total infection') were inferred within a Bayesian framework. Incidence rates relate to the number of new infections per 100 person-years for the population at risk of (primary, reinfection and total) infection. Findings: In 2022-23, the model-estimated total, re-infection and primary infection incidence rates were 3.4 per 100 person-years (95% credible interval (CrI):2.6-4.3), 1.9 (1.3-2.6), and 4.1 (3.0-5.4), respectively. For total new infections, the model-estimated incidence rate decreased by 51% from 7.0 per 100 person-years in 2015-16 to 3.4 in 2022-23 (relating to an absolute decrease of 3.6 per 100 person-years; 95% CrI: 2.0-5.3). Between 2015-16 and 2022-23, model-estimated re-infection and primary infection incidence rates decreased by 78% and 40%, respectively. Interpretation: Over a period when direct-acting antiviral therapy was scaled-up in Scotland, major reductions in the incidence of primary infection, reinfection, and total HCV infection were evident, indicating that the WHO target is within reach, for a relatively high-risk population of PWID. Funding: Public Health Scotland (for NESI); NIHR HPRU in Behavioural Science and Evaluation; NIHR Programme Grants for Applied Research programme (reference number RP-PG-0616-20008).

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