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

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.