Estimated prevalence of undiagnosed HCV infected individuals in Italy: A mathematical model by route of transmission and fibrosis progression

Background: The universal treatment of diagnosed patients with chronic HCV infection has been widely con-ducted in Italy since 2017. However, the pool of individuals diagnosed but yet to be treated in Italy has been estimated to end around 2025, leaving a significant proportion of infected individuals undiagnosed/without care. Estimates of this population are currently unknown. Methods: A probabilistic modelling approach was applied to estimate annual historical HCV incident cases by their age-group (0-100 years) distribution from available literature and Italian National database (1952 to October 2019). Viraemic infection rates were modelled on the main infection routes in Italy: people who inject drugs (PWID), tattoos, sexual transmission, glass syringe use, blood transfusion and vertical transmission. Annual liver fibrosis stage transition probabilities were modelled using a Markov model. The number of HCV viraemic asymptomatic (fibrosis stage F0-F3:potentially undiagnosed/unlinked to care) and symptomatic (fibrosis stage F4: potentially linked to care) individuals was estimated. Results: By October 2019, total viraemic HCV individuals in Italy (excluding treated patients since 1992) were estimated to be 410,775 (0.68 % of current population of Italy; 95 % CI: 0.64-0.71%, based on the current Italian population), of which 281,809 (0.47 %; 95 % CI:0.35-0.60%) were fibrosis stage F0-F3. Among different high risk groups in stage F0-F3, the following distribution was estimated: PWID; 52.0 % (95 % CI:37.9-66.6 %), tattoo; 28.8 % (95 % CI:23-32.3 %), sexual transmission; 12.0 % (95 % CI:9.6-13.7 %), glass syringe and transfusion; 6.4 % (95 % CI:2.4-17.8 %), and vertical transmission; 0.7 % (95 % CI:0.4-1.2 %). Conclusion: Under the assumption that most untreated HCV-infected individuals with stage F0-F3 are undiag-nosed, more than 280,000 individuals are undiagnosed and/or unlinked to care in Italy. Marked heterogeneity across the major routes of HCV transmission was estimated. This modelling approach may be a useful tool to characterise the HCV epidemic profile also in other countries, based on country specific epidemiology and HCV main transmission routes.