Trial of AAV5-hFIX in Severe or Moderately Severe Hemophilia B

Inclusion Criteria: 1. Male 2. Age ≥ 18 years 3. Patients with congenital hemophilia B classified as one of the following: * Known severe FIX deficiency with plasma FIX activity level \< 1% and a severe bleeding phenotype defined by one of the following: * Currently on prophylactic FIX replacement therapy for a history of bleeding * Currently on on-demand therapy with a current or past history of frequent bleeding defined as four or more bleeding episodes in the last 12 months or chronic hemophilic arthropathy (pain, joint destruction, and loss of range of motion) in one or more joints * Known moderately severe FIX deficiency with plasma FIX activity level between ≥ 1% and ≤ 2% and a severe bleeding phenotype defined by one of the following: * Currently on prophylactic FIX replacement therapy for a history of bleeding * Currently on on-demand therapy with a current or past history of frequent bleeding defined as four or more bleeding episodes in the last 12 months or chronic hemophilic arthropathy (pain, joint destruction, and loss of range of motion) in one or more joints 4. More than 150 previous exposure days of treatment with FIX protein. 5. Acceptance to use a condom during sexual intercourse in the period from Investigational Medicinal Product (IMP) administration until AAV5 has been cleared from semen, as evidenced by the central laboratory from negative analysis results for at least 3 consecutively collected semen samples (this criterion is applicable also for subjects who are surgically sterilized) 6. Following receipt of verbal and written information about the trial, the subject has provided signed informed consent before any trial related activity is carried out. Exclusion Criteria: 1. History of FIX inhibitors measured to be ≥ 0.6 Bethesda Units (BU)/mL 2. FIX inhibitors ≥ 0.6 BU/mL at Visit 1 (measured by the local laboratory) 3. Neutralizing antibodies against AAV5 at Visit 1 (measured by the central laboratory) 4. Visit 1 laboratory values (measured by the central laboratory): * alanine aminotransferase \> 2 times upper normal limit * aspartate aminotransferase \> 2 times upper normal limit * total bilirubin \> 2 times upper normal limit * alkaline phosphatase \> 2 times upper normal limit * creatinine \> 1.5 times upper normal limit 5. Positive HIV serological test at Visit 1, not controlled with anti-viral therapy as shown by cluster of differentiation 4+ counts ≤ 200 per μL or by a viral load of \>200 copies per mL (measured by the central laboratory) 6. Active infection with Hepatitis B or C virus as reflected by Hepatitis B Surface Antigen (HBsAg), Hepatitis B extracellular Antigen (HBeAg), Hepatitis B Virus DeoxyriboNucleic Acid (HBV DNA) or Hepatitis C Virus RiboNucleic Acid (HCV RNA) positivity, respectively, at Visit 1 (measured by the central laboratory). 7. History of Hepatitis B or C exposure, currently controlled by antiviral therapy 8. Any coagulation disorder other than hemophilia B 9. Thrombocytopenia, defined as a platelet count below 50 × 10E9 / L, at Visit 1 (measured by the central laboratory) 10. Body mass index \< 16 or ≥ 35 kg/m2 11. Planned surgery for the initial 6 months after IMP administration in this trial 12. Previous arterial or venous thrombotic event (e.g. acute myocardial infarction, cerebrovascular disease and venous thrombosis) 13. Active severe infection or any other significant concurrent, uncontrolled medical condition including, but not limited to, renal, hepatic, haematological, gastrointestinal, endocrine, pulmonary, neurological, cerebral or psychiatric disease, alcoholism, drug dependency or any other psychological disorder evaluated by the investigator to interfere with adherence to the protocol procedures or with the degree of tolerance to the IMP 14. Known significant medical condition including disseminated intravascular coagulation, fibrinolysis and liver fibrosis which, in the opinion of the investigator, may confound, contraindicate or limit the interpretation of either safety or efficacy data 15. Known history of an allergic reaction or anaphylaxis to FIX products 16. Known uncontrolled allergic conditions or allergy/hypersensitivity to any component of the IMP excipients 17. Previous gene therapy treatment and/or previous participation in a gene therapy clinical trial 18. Receipt of an experimental agent within 60 days prior to Visit 1 19. Current participation or anticipated participation within one year after IMP administration in this trial in any other interventional clinical trial involving drugs or devices.