The primary aim of this study is to calculate the incidence of HCV reinfection after successful DAA treatment among people who have recently injected drugs. The secondary aim is to identify factors associated with reinfection in this population.

Individuals with active injecting drug abuse with a chronic HCV infection who have achieved end of treatment response (ETR; defined as non-detectable HCV RNA at end of treatment) to any interferon-free DAA combination will be included in this multicenter interventional study.

This protocol is an adapted version of the protocol 'Hepatitis C reinfection after successful directly acting antiviral treatment: A North-Atlantic multicenter interventional study'. The investigators will plan this study so that the data are similar to this protocol and results can be compared within this alliance of hepatologists with special interest in care for substance users.

Study schedule: Data collection will be performed according to the study schedule shown in Table 1. Following assessment for SVR12, participants will be followed for 2 years at 6 months' intervals at the discretion of each study site.

Inclusion Criteria:

• Informed consent
• Age 18 years or older
• Injecting drug use within 6 months before start of treatment
• Achieved an ETR following at least eight weeks of DAA treatment
• Completed DAA treatment no more than 6 months prior to inclusion
• Blood sample drawn within 6 months pre-treatment stored at -70° C

Exclusion Criteria:

• Patients not fulfilling the inclusion criteria.