Phase 3, Open-label, Single-dose Study of CSL222 in Adolescent Male Subjects (≥ 12 to < 18 Years of Age) With Severe or Moderately Severe Hemophilia B

This is a phase 3, prospective, open-label, single-arm, single-dose, multicenter study investigating the efficacy, safety, and tolerability of CSL222 (AAV5-hFIXco-Padua) in adolescent male participants with severe or moderately severe hemophilia B.

Inclusion Criteria: * Key Inclusion Criteria for the Lead-in Period: Assigned male sex at birth * Aged ≥138 months (11 years and 6 months) to less than (\ 2 × the upper limit of normal (ULN). * Alanine aminotransferase (ALT) \> 2 × the ULN. * Aspartate aminotransferase (AST) \> 2 × the ULN. * Alkaline phosphatase (ALP) \> 2 × the ULN. * Serum creatinine \> 2 × the ULN. * Hemoglobin \< 8 g/dL. * Any condition other than hemophilia B resulting in an increased bleeding tendency. * Thrombocytopenia, defined as a platelet count below 50 × 10\^9/L, at screening (based on central laboratory results). * Any uncontrolled or untreated infection (human immunodeficiency virus, hepatitis C, etc) or any other significant concurrent, uncontrolled medical condition, as evaluated by the investigator, including, but not limited to renal, hepatic, cardiovascular, hematological, 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 Clinical Study Protocol procedures or with the degree of tolerance to CSL222. * Additional Key Exclusion Criteria for the Treatment Period: Positive FIX inhibitor test at Visit L-Final (based on central laboratory results) * AAV5 NAb titer \> 1:900 as assessed at Visit LX (last visit before Visit L-Final). * Visit L-Final laboratory values (based on central laboratory results) of: * Total bilirubin \> 2 × the ULN * ALT \> 2 × the ULN. * AST \> 2 × the ULN. * ALP \> 2 × the ULN. * Serum creatinine \> 2 × the ULN. * Hemoglobin \< 8 g/dL. * Thrombocytopenia, defined as a platelet count below 50 × 10\^9/L, at Visit L-Final (based on central laboratory results).