Study to Evaluate the Pharmacokinetics, Safety, and Antiviral Activity of the Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Alafenamide (E/C/F/TAF) Single Tablet Regimen (STR) in HIV-1 Infected Antiretroviral Treatment-Naive Adolescents and Virologically Suppressed Children

The primary objectives of Cohort 1 are to evaluate the steady state pharmacokinetics (PK) for elvitegravir (EVG) and tenofovir alafenamide (TAF) and confirm the dose of the elvitegravir/cobicistat/emtricitabine/tenofovir alafenamide (E/C/F/TAF) STR (Part A) and to evaluate the safety and tolerability of E/C/F/TAF STR through Week 24 (Part B) in human immunodeficiency virus - 1 (HIV-1) infected, antiretroviral (ARV) treatment-naive adolescents. The primary objectives of Cohort 2 are to evaluate the PK of EVG and TAF (Part A), and to evaluate the safety and tolerability of E/C/F/TAF through Week 24 (Part B) in virologically suppressed HIV-1 infection children 6 to \= 25 kg. The primary objectives of Cohort 3 are to evaluate the PK of EVG and TAF and confirm the dose of the STR, and to evaluate the safety and tolerability of E/C/F/TAF low dose (LD) STR in virologically suppressed HIV-1 infected children ≥ 2 years of age and weighing ≥ 14 to \< 25 kg.