Introduction The historical incidence of hepatitis C virus (HCV) infection in Scotland's injecting drug user (IDU) population is unknown. From a public health perspective, accurate data on incidence rates over time would improve our understanding of the effects of harm reduction measures initiated in the late 1980s (needle/syringe exchanges, methadone treatment) on the transmission of HCV among Scotland's IDUs. Because HCV-antibody testing only commenced in 1991, and positive diagnoses are often made many years after infection, trends in incidence cannot easily be inferred from trends in the time-series of diagnosed cases.

Methods We applied back-calculation methods to reconstruct HCV incidence using data available on the national HCV Diagnosis database (1991–2009; n=25 000), estimates from the literature regarding time to seroconversion and mortality rates, and the estimated distribution of time between injection debut and diagnosis, derived via record-linkage between the HCV Diagnosis and the Scottish Drugs Misuse databases.

Results Approximately 50 000 IDUs were estimated to have been infected with HCV during 1960–2009. The shape of the incidence curve varied with region, but a peak in annual incidence was estimated to have occurred around 1993. The model projected approximately 1700 new HCV diagnoses per year over the coming 5 years. Sensitivity analyses were also conducted to explore assumptions regarding mortality rate and the proportion of infected IDUs that are never diagnosed.

Conclusion These incidence data are important for evaluation of the impact of harm reduction initiatives, for informing national public health planning, and for projecting the future burden of HCV-related severe liver disease.