This is a Phase 2b/3 study designed to evaluate the safety and efficacy of chronic treatment with brelovitug (a.k.a BJT-778; BTG) for chronic hepatitis delta virus (HDV) infection. The comparator in this study will be 24-weeks of delayed treatment. During the 24-weeks of delayed treatment, participants will complete the same visits and assessments as those randomized to initiate brelovitug immediately. At the completion of 24-week delayed treatment period, all participants will start treatment with brelovitug.

Study will consist of 3 study arms. Approximately 150 participants will be randomized 2:2:1 to one of the following treatment arms:

• Arm 1: Participants randomized to Arm 1 will receive brelovitug 300 mg subcutaneously once weekly.
• Arm 2: Participants randomized to Arm 2 will receive brelovitug 900 mg subcutaneously once every 4 weeks.
• Arm 3: Participants randomized to Arm 3 will attend study clinic visits and delay treatment with brelovitug. At Week 24, all participants will receive brelovitug 300 mg subcutaneously once weekly.

Inclusion Criteria:

• Willing and able to provide written informed consent.
• Chronic HDV infection
• HDV RNA >500 IU/mL at Screening.
• Abnormal ALT (>upper limit of normal) at Screening.
• Willing to take or already taking HBV nucleos(t)ide therapy

Exclusion Criteria:

• Pregnant or nursing females.
• Unwilling to comply with contraception requirements during the study.
• Difficulty with blood collection and/or poor venous access for the purposes of phlebotomy
• Presence of other liver disease(s) (does not include HBV or HDV infection) such as non-alcoholic steatohepatitis (NASH), alcohol associated hepatitis, cholestatic liver disease, hepatocellular carcinoma.
• Clinical hepatic decompensation (i.e., ascites, encephalopathy variceal hemorrhage).
• Solid organ or bone marrow transplantation Note: other protocol defined Inclusion/Exclusion criteria may apply.