This study is intended to confirm the efficacy, safety, pharmacokinetic (PK) profile, and the durability of hepatitis B virus surface antigen (HBsAg) suppression observed with bepirovirsen for 24 weeks (with loading doses) as compared to the placebo arm. This study will have 4 stages: a) Double-blind treatment (bepirovirsen or placebo) for 24 weeks. b) Nucleos(t)ide analogue (NA) treatment for 24 weeks. c) NA cessation stage OR Continue NA for 24 weeks. d) Durability of response and follow up for further 24 weeks for participants who stopped NA treatment at Week 48. The arms will be stratified based on HBsAg level (HBsAg greater than or equal to [≥] 100 international unit per milliliter [IU/mL] to less than or equal [≤]1000 IU/mL or greater than [>] 1000 IU/mL to ≤3000 IU/mL) at screening. The total duration of the study, including screening (up to 60 days), the double-blind treatment stage (24 weeks), the On NA only stage (24 weeks), and the NA cessation and durability stages (48 weeks) is up to approximately 104 weeks at maximum for each participant.

Inclusion Criteria:

• Participants who have documented chronic HBV infection ≥6 months prior to screening and currently receiving stable NA therapy defined as no changes to their NA regimen from at least 6 months prior to Screening and with no planned changes to the stable regimen over the duration of the study.
• Plasma or serum HBsAg concentration >100 IU/mL, but no greater than ≤3000 IU/mL.
• Plasma or serum HBV DNA concentration must be adequately suppressed, defined as plasma or serum HBV DNA <90 IU/mL.
• Alanine aminotransferase (ALT) ≤2 × upper limit of normal (ULN).
• Participants who are willing and able to cease their NA treatment in accordance with the protocol.

Exclusion Criteria:

• Clinically significant abnormalities, aside from chronic HBV infection in medical history (e.g., moderate severe liver disease other than chronic HBV, acute coronary syndrome within 6 months of screening, major surgery within 3 months of screening, significant/unstable cardiac disease, uncontrolled diabetes, bleeding diathesis or coagulopathy) or physical examination.

Co-infection with:

a) Current history of Hepatitis C infection or participants that have been cured for <12 months at the time of screening b) Human immunodeficiency virus (HIV), c) Hepatitis D virus.

• History of or suspected liver cirrhosis and/or evidence of cirrhosis.
• Diagnosed or suspected hepatocellular carcinoma.
• History of malignancy within the past 5 years except for specific cancers that are cured by surgical resection (e.g., skin cancer). Participants under evaluation for possible malignancy are not eligible.
• History of vasculitis or presence of symptoms and signs of potential vasculitis (e.g., vasculitic rash, skin ulceration, repeated blood detected in urine without identified cause) or history/presence of other diseases that may be associated with vasculitis condition (e.g., systemic lupus erythematosus, rheumatoid arthritis, relapsing polychondritis, mononeuritis multiplex).
• History of extrahepatic disorders possibly related to HBV immune conditions (e.g., nephrotic syndrome, any type of glomerulonephritis, polyarteritis nodosa, cryoglobulinemia, uncontrolled hypertension).
• History of alcohol or drug abuse/dependence.
• Currently taking, or took within 3 months of screening, any immunosuppressing drugs (e.g., prednisone), other than a short course of therapy (≤2 weeks) or topical/inhaled steroid use.
• Participants to whom immunosuppressive treatment, including therapeutic doses of steroids is contraindicated, should not be considered for enrolment in the study.
• Currently taking, or has taken within 12 months of Screening, any interferon containing therapy.
• Participants requiring anti coagulation therapies (e.g., warfarin, Factor Xa inhibitors) or anti-platelet agents (like clopidogrel or aspirin) unless treatment can safely be discontinued throughout duration of the study, by the discretion of the investigator. Occasional use is permitted.
• Prior treatment with any oligonucleotide or siRNA within 12 months prior to the first dosing day.
• Prior treatment with bepirovirsen.

Qualified researchers may request access to anonymized individual patient-level data (IPD) and related study documents of the eligible studies via the Data Sharing Portal. Details on GSK's data sharing criteria can be found at: https://www.gsk.com/en-gb/innovation/trials/data-transparency/

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or terminated asset(s) across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension may be granted, when justified, for up to 6 months.

This will be a double blinded study in which investigators/site staff and the Sponsor will remain blinded to treatment assignment.