Efficacy and Safety of CSL222 (Etranacogene Dezaparvovec) Gene Therapy in Adults With Hemophilia B With Pretreatment Adeno-associated Virus Serotype 5 (AAV5) Neutralizing Antibodies (Nabs)

The purpose of this study is to assess the risk of bleeding due to failure of expected pharmacological action of CSL222 in adults with severe or moderately severe hemophilia B with detectable pretreatment AAV5 Nabs.

Inclusion Criteria: * Considered legally an adult, as defined by country regulations. * Has congenital hemophilia B with known severe or moderately severe FIX deficiency (less than or equal to \[\) 150 previous exposure days to FIX replacement therapy. * Has been on stable FIX prophylaxis for at least 2 months before Screening. * Has demonstrated capability to independently, accurately, and in a timely manner complete the eDiary during the Lead-in Period, as judged by the investigator. * Acceptance to adhere to contraception guidelines. * Able to provide informed consent after receipt of verbal and written information about the study. * Investigator believes that the participant (or the participant's legally acceptable representative\[s\]) understands the nature, scope, and possible consequences of the study and is able to adhere to the study procedures. Exclusion Criteria: * History of FIX inhibitors or positive FIX inhibitor test at Prescreening, Screening or Visit L-Final (based on central laboratory results). * Screening or Visit L-Final laboratory values (based on central laboratory results) of total bilirubin \> 2 × the upper limit of normal (ULN) (except if caused by Gilbert's syndrome). * Screening or Visit L-Final laboratory values (based on central laboratory results) of any of the following laboratory abnormalities: 1. ALT \> 2 × the ULN 2. AST \> 2 × the ULN 3. Alkaline phosphatase \> 2 × the ULN 4. Serum creatinine \> 2 × the ULN 5. Hemoglobin less than (\