Analysing HCV incidence trends in people who inject drugs using serial behavioural and seroprevalence data: A modelling study

•Seroprevalence data that include time at risk can provide estimates of the rate at which new infections occur.•This paper explores flexible models to estimate the rate of first infection of HCV over time.•The rate of first infection was found to increase over 2011–2020 in recent initiates, but was stable in those injecting for more than 5 years.•Results depend on modelling assumptions and limitations of surveillance data. However, this method provides an additional tool for tracking WHO metrics on incidence, which is difficult to monitor directly. The introduction of new direct-acting antivirals for hepatitis C virus (HCV) infection, has enabled the formulation of a HCV elimination strategy led by the World Health Organisation (WHO). Guidelines for elimination of HCV target a reduction in incidence, but this is difficult to measure and needs estimating. Serial cross-sectional bio-behavioural sero-surveys provide information on an individual's infection status and duration of exposure and how these change over time. These data can be used to estimate the rate of first infection through appropriate statistical models. This study utilised updated HCV seroprevalence information from the Unlinked Anonymous Monitoring survey, an annual survey of England, Wales and Northern Ireland monitoring the prevalence of blood borne viruses in people who inject drugs. Flexible parametric and semiparametric approaches, including fractional polynomials and splines, for estimating incidence rates by exposure time and survey year were implemented and compared. Incidence rates were shown to peak in those recently initiating injecting drug use at approximately 0.20 infections per person-year followed by a rapid reduction in the subsequent few years of injecting to approximately 0.05 infections per person-year. There was evidence of a rise in incidence rates for recent initiates between 2011 and 2020 from 0.17 infections per person-year (95 % CI, 0.16–0.19) to 0.26 infections per person-year (0.23–0.30). In those injecting for longer durations, incidence rates were stable over time. Fractional polynomials provided an adequate fit with relatively few parameters, but splines may be preferable to ensure flexibility, in particular, to detect short-term changes in the rate of first infection over time that may be a result of treatment effects. Although chronic HCV prevalence has declined with treatment scale up over 2016–2020, there is no evidence yet of a corresponding fall in the ra