A Study of JNJ-73763989 + Nucleos(t)Ide Analog in Participants Co-Infected With Hepatitis B and Hepatitis D Virus

Inclusion Criteria:

   - Medically stable based on physical examination, medical history, vital signs,
   electrocardiogram (ECG) at screening

   - Chronic hepatitis B virus (HBV) and hepatitis D virus (HDV) co-infection with
   documentation at least 6 months prior to screening

   - For Part 1: hepatitis D RNA (HDV RNA) greater than or equal to (>=) 1000 international
   units per milliliter (IU/mL) at screening. For Part 2: must have HDV RNA values >= 500
   IU/mL, and must have hepatitis B surface antigen (HBsAg) values less than or equal to
   (<=) 10000 IU/mL at screening or HDV RNA values at screening are <= 100000 IU/mL

   - Alanine aminotransferase (ALT) greater than upper limit normal (ULN) but less than 10
   times (ULN)

   - Body mass index (BMI) between 18.0 and 35.0 kilogram per meter square (kg/m^2),
   extremes included

   - Highly effective contraceptive measures in place for female participants of
   childbearing potential or male participants with female partners of childbearing
   potential

   - Non-cirrhotic participants and participants with compensated cirrhosis (Child Pugh
   class A) at screening (Part 1) and participants must have absence of cirrhosis and
   platelet count of >= 140000 per deciliter (dL) for enrollment into Part-2

Exclusion Criteria:

   - Evidence of infection with hepatitis A, C, or E virus infection or evidence of human
   immunodeficiency, virus type 1 (HIV-1) or HIV-2 infection at screening

   - History or evidence of clinical signs/symptoms of hepatic decompensation including but
   not limited to: portal hypertension, ascites, hepatic encephalopathy, esophageal
   varices or any laboratory abnormalities indicating a reduced liver function as defined
   in the protocol

   - Evidence of liver disease of non-HBV/HDV etiology

   - Signs of hepatocellular carcinoma (HCC)

   - Significant laboratory abnormalities as defined in the protocol at screening

   - Participants with a history of malignancy within 5 years before screening

   - Abnormal sinus rhythm or ECG parameters at screening as defined in the protocol

   - History of or current cardiac arrhythmia or history or clinical evidence of
   significant or unstable cardiac disease

   - Participants with any current or previous illness for which, in the opinion of the
   investigator and/or sponsor, participation would not be in the best interest of the
   participant

   - History of or current clinically significant skin disease or drug rash

   - Participants with known allergies, hypersensitivity, or intolerance to JNJ-3989 or its
   excipients or excipients of the placebo content

   - Contraindications to the use of entecavir (ETV), tenofovir disoproxil, or tenofovir
   alafenamide (TAF) per local prescribing information

   - Participants who have taken any therapies disallowed per protocol

   - Female participants who are pregnant, or breast-feeding, or planning to become
   pregnant while enrolled in this study or within 90 days after the last dose of study
   intervention

   - Male participants who plan to father a child while enrolled

   - Participants who had or planned major surgery, (example, requiring general anesthesia)
   or who have received an organ transplant

   - Vulnerable participants (example, incarcerated individuals, individuals under a legal
   protection measure)