Study To Evaluate Emtricitabine/Tenofovir Alafenamide (F/TAF) in Human Immunodeficiency Virus 1 (HIV-1) Infected Children and Adolescents Virologically Suppressed on a 2- Nucleoside/Nucleotide Reverse Transcriptase Inhibitor (2-NRTI)-Containing Regimen

The primary objective of this study is to confirm the TAF dose and to evaluate the pharmacokinetics (PK) of TAF, safety, and tolerability of F/TAF in HIV-1 infected children and adolescents virologically suppressed (defined as having \< 50 copies/mL of HIV-1 ribonucleic acid \[RNA\] for a period of at least 6 months) while on a stable NRTI containing regimen.

Cohorts 2, 3, and 4 will be on a boosted protease inhibitor (PI) or any other 3rd antiretroviral (ARV) agent and will switch their current 2-NRTI-containing regimen to open-label F/TAF while continuing their boosted PI or 3rd agent through 48 weeks. A minimum of 10 participants each in Groups 1 and 2 of Cohort 2, and Cohorts 3 and 4, who are on boosted-atazanavir (ATV) as their 3rd ARV agent will be enrolled. Participants in Cohort 2, Group 1 receive boosted PI agents only. Cohorts 2, 3, and 4 will be enrolled by cohort into a two-part study (Parts A and B).

After completion of 48 weeks, all participants will be given the option to participate in an extension phase of the study. Gilead will provide F/TAF until a) The participant turns 18 years old and F/TAF is commercially available for use in adults in the country in which the participant is enrolled or b), F/TAF becomes commercially available for pediatric use in the country in which the participant is enrolled or c), Gilead Sciences elects to terminate development of F/TAF in the applicable country.

Eligibility Criteria:
Key Inclusion Criteria:

* HIV-1 infected male and female adolescents and children aged 1 month to \< 18 years at baseline/Day 1 (according to requirements of the enrolling cohort)
* Must be able to give written assent prior to any screening evaluations
* Parent or guardian able to give written informed consent prior to any screening evaluations and willing to comply with study requirements
* Body weight at screening as follows:

  * Cohort 1: ≥ 35 kg
  * Cohort 2, Group 1: ≥ 25 kg
  * Cohort 2, Group 2: 17 kg to \< 25 kg
  * Cohort 3: to be updated per a protocol amendment
  * Cohort 4: to be updated per a protocol amendment
* Currently on a stable 2-NRTI containing regimen that includes a 3rd ARV agent for ≥ 6 consecutive months prior to screening
* Plasma HIV-1 RNA levels \< 50 copies/mL for ≥ 6 consecutive months preceding the screening visit
* No opportunistic infection within 30 days of study entry (at baseline/Day 1)
* A negative serum β-human chorionic gonadotropin (HCG) pregnancy test is required for females of childbearing potential only

Key Exclusion Criteria:

* An acquired immunodeficiency syndrome (AIDS) - indicator condition with onset within 30 days prior to screening
* Life expectancy of \< 2 years
* Active, serious infections (other than HIV-1 infection) requiring parenteral antibiotic or antifungal therapy within 30 days prior to baseline/Day 1
* Evidence of active pulmonary or extra-pulmonary tuberculosis disease within 3 months of the screening visit
* Active hepatitis C virus (HCV) infection defined as positive for HCV antibody and having detectable HCV RNA
* Positive hepatitis B surface antigen or other evidence of active hepatitis B virus (HBV) infection.
* Have any serious or active medical or psychiatric illness which, in the opinion of the Investigator, would interfere with treatment, assessment, or compliance with the protocol.
* Pregnant or lactating females
* Have history of significant drug sensitivity or drug allergy
* Have previously participated in an investigational trial involving administration of any investigational agent, other than tenofovir, within 30 days prior to the study dosing

NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply.