Study of Tenofovir Alafenamide (TAF) in Children and Teen Participants With Chronic Hepatitis B Virus Infection

The goals of this clinical study are to compare the effectiveness, safety and tolerability of study drug, tenofovir alafenamide (TAF), versus placebo in teens and children with CHB and to learn more about the dosing levels in children.

Key Inclusion criteria: * Males and non-pregnant, non-lactating females * Weight at screening as follows: * Cohort 1 = ≥ 35 kg (≥ 77 lbs) * Cohort 2 Group 1 = ≥ 25 kg (≥ 55 lbs) * Cohort 2 Group 2 = ≥ 14 kg to \< 25 kg (≥ 30 lbs to \<55 lbs) * Cohort 2 Group 3 = ≥ 10 kg to \< 14 kg (≥ 22 lbs to \< 30 lbs) or * 14 kg to \< 25 kg (≥ 30 lbs to \< 55 lbs) * Willing and able to provide written informed consent/assent (child and parent/legal guardian) * Documented evidence of CHB (eg, HBsAg-positive for ≥ 6 months) * HBeAg-positive, or HBeAg-negative, chronic HBV infection with all of the following: * Screening HBV DNA ≥ 2 × 10\^4 IU/mL * Screening serum ALT \> 45 U/L (\> 1.5 × ULN: 30 U/L) and ≤ 10 × ULN (by central laboratory range) * Treatment-naive or treatment-experienced will be eligible for enrollment. * Estimated creatinine clearance (CLCr) ≥ 80 mL/min/1.73m\^2 (using the Schwartz formula) * Normal ECG Key Exclusion criteria: * Females who are pregnant or breastfeeding * Males and females of reproductive potential who are unwilling to use an "effective", protocol-specified method(s) of contraception during the study. * Coinfection with hepatitis C virus (HCV), HIV, or hepatitis D virus (HDV) * Evidence of hepatocellular carcinoma (Note: if screening alpha-fetoprotein (AFP) is \< 50 ng/mL no imaging study is needed; however, if the screening AFP is \> 50 ng/mL an imaging study is required) * Any history of, or current evidence of, clinical hepatic decompensation * Abnormal hematological and biochemical parameters * Chronic liver disease of non-HBV etiology (e.g., hemochromatosis, alpha-1 antitrypsin deficiency, cholangitis) * Received solid organ or bone marrow transplant * Currently receiving therapy with immunomodulators (eg, corticosteroids), or immunosuppressants * Significant renal, cardiovascular, pulmonary, or neurological disease in the opinion of the Investigator * Malignancy within the 5 years prior to screening. Individuals under evaluation for possible malignancy are not eligible. * Current alcohol or substance abuse judged by the investigator to potentially interfere with subject compliance. Note: Other protocol defined Inclusion/Exclusion criteria may apply.