Inclusion Criteria:
1. Adult males or females aged ≥18 to ≤65 years at screening (according to country/local regulations)
2. BMI ≤35 kg/m2
3. Able to provide informed consent indicating they understand the purpose of, and procedures required, for the study and are willing to participate
4. If female, willing not to become pregnant up to 8 weeks after the last dose of study vaccine and up to 5 months after the last dose of nivolumab
5. If female: Not pregnant or breast feeding and one of the following:
* Of non-childbearing potential (i.e., women who have had a hysterectomy or tubal ligation or are postmenopausal, as defined by no menses in ≥1 year and without an alternative medical cause)
* Of childbearing potential but agrees to practice highly effective contraception for 4 weeks prior to study vaccine and 8 weeks after VTP-300 and 5 months after the last dose of nivolumab. Highly effective methods of contraception include one or more of the following:
* Male partner who is sterile (medically effective vasectomy) prior to the female participant's entry into the study and is the sole sexual partner for the female participant
* Combined (oestrogen and progestogen-containing) hormonal contraception associated with inhibition of ovulation: oral, intravaginal or transdermal
* Progestogen-only hormonal contraception associated with inhibition of ovulation: oral, injectable or implantable
* An intrauterine device
* Bilateral tubal occlusion
* Abstinence from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent
6. Documented evidence of CHB infection (e.g., HBsAg positive ≥6 months with detectable HBsAg levels at screening; both HBeAg+ and HBeAg- allowed)
7. Receipt of only either entecavir, tenofovir (tenofovir alafenamide fumarate or tenofovir disoproxil fumarate) or besifovir for at least 6 months before screening
8. HBV-DNA viral load ≤ 1,000 IU/mL
9. HBsAg levels \> 10 and ≤ 4,000 IU/mL
Exclusion Criteria:
1. Presence of any significant acute or chronic, uncontrolled medical or psychiatric illness in the opinion of the investigator would affect the safety of the participant or the evaluation of the data or interfere with adherence to the study requirements
2. Medical history that is thought to increase the participant's risk of reaction to a vaccine, including but not limited to capillary leak syndrome; transverse myelitis, Guillain Barré syndrome, thrombosis with thrombocytopenia syndrome (also termed vaccine-induced thrombotic thrombocytopenia); heparin-induced thrombocytopenia HCV RNA positive
3. HIV antibody positive and active hepatitis C (antibody positive and then DNA positive)
4. Co-infection with hepatitis D virus (HDV)
5. Documented cirrhosis or advanced fibrosis indicated by a liver biopsy within 6 months prior to Day 0 (Metavir activity grade A4 and stage F4; Ishak stages 5 - 6).
6. In the absence of a documented liver biopsy, either 1 of the following (not both):
* Screening Fibroscan with a result \>9 kilopascals (kPa) (or the equivalent) within ≤ 6 months of screening, OR
* Both screening FibroTest \>0.48 and aspartate aminotransferase (AST) to platelet ratio index (APRI) of \>1.
7. ALT \>3 x ULN, or INR \>1.5 unless the participant was stable on an anticoagulant regimen affecting INR, albumin \<3.2 g/dL, direct bilirubin \>1.5 x ULN, platelet count \<100,000/µL.
8. A history of liver decompensation (e.g., ascites, encephalopathy or variceal haemorrhage)
9. Prior hepatocellular carcinoma
10. Chronic liver disease of a non-HBV aetiology. (Note that Gilbert's syndrome, asymptomatic gallstones and non-alcoholic fatty liver not associated with steatohepatitis are not exclusions)
11. History or evidence of autoimmune disease or known immunodeficiency of any cause except history of autoimmune thyroiditis if the participant is stable on replacement therapy
12. Evidence of interstitial lung disease, active pneumonitis, myocarditis or a history of myocarditis
13. Prolonged therapy with immunomodulators (e.g., corticosteroids such as prednisone \>10 mg/day) or biologics (e.g., monoclonal antibodies, IFN) within 3 months of Day 1. Inhaled, intra-articular, intra-bursal or topical corticosteroids are allowed. Physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency is allowed.
14. Receipt of immunoglobulin or other blood products within 3 months prior to Day 1
15. Receipt of any investigational drug or vaccine within 3 months prior to Day 1
16. Receipt of any non-oral adenoviral-based vaccine within 3 months prior to administration of ChAdOx1-HBV on Day 1
17. Severe reaction to any vaccine, requiring medical attention
18. Receipt of any live vaccines within 30 days prior to Day 1
19. Receipt of any inactivated non-live vaccines (e.g., mRNA, inactivated vaccines, toxoid vaccines) within 14 days prior to Day 1
20. History of severe hypersensitivity or anaphylactic reactions likely to be exacerbated by any component of VTP-300 or nivolumab, including severe allergy to egg
21. Malignancy within 5 years prior to screening with the exception of basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 years. Participants under evaluation for possible malignancy are not eligible
22. Current alcohol or substance abuse judged by the investigator to potentially interfere with participant safety or compliance
23. Any laboratory test which is abnormal, and which is deemed by the investigator to be clinically significant
24. Any other finding that, in the opinion of the investigator, deems the participant unsuitable for the study
