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

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 (\<) 206 months (17 years and 2 months) at the time of informed consent / assent. * Congenital hemophilia B with known severe or moderately severe FIX deficiency (less than or equal to \[≤\] 2% of normal circulating FIX) for which the participant has been on continuous FIX prophylaxis. * On stable continuous FIX prophylaxis for at least 2 months before Screening. * Minimum of 75 previous exposure days of treatment with FIX protein before Screening. * Additional Key Inclusion Criteria for the Treatment Period: Completed the Lead-in Period: minimum of 6 months (26 weeks) of lead-in data collected and eligibility has been confirmed. * Aged ≥ 12 to \< 18 years at the time of CSL222 treatment. Exclusion Criteria: * Key Exclusion Criteria for the Lead-in Period: History of FIX inhibitors or positive FIX inhibitor test at Screening (based on central laboratory results). * Screening laboratory values (based on central laboratory results): * Total bilirubin \> 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 p…