Rationale: Hepatitis delta virus (HDV) is a defective RNA virus that requires presence of hepatitis B virus (HBV) to complete virion assembly and secretion. HBV-HDV coinfection ("hepatitis delta") has been associated with severe liver injury that may result in rapid progression to cirrhosis and hepatic decompensation, as well as a higher risk of liver cancer when compared to patients with HBV mono-infection. Given the low incidence of hepatitis D, experience in caring for individuals with hepatitis delta is limited and management practices vary.

Objective: Generate prospective follow-up data to increase our understanding of this rare disease.

Study design: Prospective observational cohort study spanning 5 years, during which we will collect standard clinical data as well as blood samples and quality of life questionnaires.

Study population: hepatitis delta patients aged ≥18 years Intervention (if applicable): not applicable Main study parameters/endpoints: Incidence of liver related events (liver cancer, (decompensation of) cirrhosis, liver transplantation) during follow-up and changes in markers of viral replication, inflammatory activity and liver stiffness over time.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The risks associated with participation can be considered negligible and the burden can be considered minimal. The only additional action that the participants must perform are the filling out of two annual quality of life questionnaires, which are considered non-invasive, and collection of 10 ml blood during regular blood sample collections

Inclusion Criteria:

• Active hepatitis delta based on a positive anti-HDV and a positive HDV-RNA test
• Patients must be ≥18 years
• Written informed consent

Exclusion Criteria:

• None

All hepatitis delta patients currently in care at the participating sites will be asked to enter the study. The exact number of eligible patients is unclear, but based on the DREAM-1 study we expect to enrol up to 150 patients.