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.
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.
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.