Hemophilia B is a genetic bleeding disorder caused by pathogenic variants (eg, mutations, deletion) in the FIX gene. HB patients have frequent and potentially life-threatening bleeding and often develop progressive physical disability and pain from chronic haemarthropathy. Current replacement therapy needs regular treatment in the life-long time, bringing heavy economic and social burdens.VGB-R04 is a novel AAV vector carrying a high specific activity factor IX variant.~This is a prospective, open, multicenter, observational lead-in study. To evaluate the efficacy and safety of current treatment regiments in patients with moderate to severe HB (FIX:C activity ≤2%). The efficacy and safety data collected in this study may be used as baseline data for subsequent related clinical trials of VGB-R04. All subjects in this study will provide informed consent and then undergo screening assessments up to 4 weeks. All subjects will undergo 26 weeks of efficacy and safety observation .Eligible subjects will be invited to a key clinical study（phase 2 study）of VGB-R04,and then encouraged to enroll in an Long-term follow-up study to evaluate the long-term safety of VGB-R04 for a total of five years.
This is a prospective, open, multicenter, observational lead-in study，to collect prospective efficacy and safety data of current replacement therapy in adult hemophilia B patients.