A Study to Evaluate Prospective Efficacy and Safety Data of Current FIX Prophylaxis Replacement Therapy in Adult Hemophilia B Subjects (FIX:C≤2%) or Current FVIII Prophylaxis Replacement Therapy in Adult Hemophilia A Subjects (FVIII:C≤1%)

Inclusion Criteria

Hemophilia B Population:

• Evidence of a signed and dated informed consent document indicating that the participant has been informed of all pertinent aspects of the study.
• Willing and able to comply with scheduled visits, FIX prophylaxis treatment plan, laboratory tests and other study procedures.
• Males ≥ 18 and 8 kPa units OR FibroTest/FibroSURE >0.48*.
• NOTE: If there is concern regarding the validity of any of the liver fibrosis test results please contact the medical monitor to discuss whether any additional testing needs to be performed (ie, either repeating any test or performing another fibrosis test). Also, note, if a participant has a known history of Gilbert's syndrome, a FibroTest cannot be used for fibrosis testing.
• Documented serological evidence of human immunodeficiency virus HIV-1 or HIV-2 with Cluster of Differentiation 4 positive (CD4+) cell count ≤200 mm3 within the last 12 months prior to screening. Participants who are HIV positive and stable, have an adequate CD4 count (>200/mm3) and undetectable viral load (<50 gc/mL) documented within the preceding 12 months, and are on an antiretroviral drug regimen are eligible to enroll. Participants who have not been tested within the prior 12 months of screening will need to be tested for HIV status at screening.
• History of chronic infection or other chronic disease that the investigator deems an unacceptable risk. In addition, any participant with conditions associated with increased thromboembolic risk such as known inherited or acquired thrombophilia, or a history of thrombotic events including but not limited to stroke, myocardial infarction, and/or venous thromboembolism, is excluded.
• Any concurrent clinically significant major disease or condition that the investigator deems unsuitable for participation or other acute or chronic medical or psychiatric condition including recent (within the past year) or active suicidal ideation or behavior or laboratory abnormality that may increase the risk associated with study participation or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the participant inappropriate for entry into this study. In addition, any participant with a history of a neoplasm (including hepatic malignancy) that required treatment (eg, chemotherapy, radiotherapy, immunotherapy), is excluded, except for adequately treated basal or squamous cell carcinoma of the skin or a surgically removed benign neoplasm not requiring chemotherapy, radiotherapy and/or immunotherapy. Any other neoplasm that has been cured by resection should be discussed between the investigator and sponsor.
• Participation in other studies if involving administration of investigational product(s) within the last 3 months prior to study entry and/or during study participation or in a previous gene therapy clinical study within the last 12 months prior to screening.
• Participants already enrolled in this lead-in study (C0371004) may be allowed to participate in the screening and baseline periods of either C0371002 or C3731003 protocols prior to their completion of the end of study visit in this lead-in study.
• Any participant who previously received fidanacogene elaparvovec (hemophilia B) or giroctocogene fitelparvovec (hemophilia A) or any AAV gene-based therapy.
• Participants using restricted therapies.
• Any participant with a planned surgical procedure requiring FIX (hemophilia B) or FVIII (hemophilia A) surgical prophylactic factor treatment in the next 24 months.
• Investigator site staff members directly involved in the conduct of the study and their family members, site staff members otherwise supervised by the investigator, or participants who are Pfizer employees, including their family members, directly involved in the conduct of the study.

NOTE: The sponsor's medical team should be contacted if there are any questions regarding any of the inclusion or exclu