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 conducted 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 undiagnosed, 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.