Study of Cobicistat-Boosted Atazanavir (ATV/co), Cobicistat-Boosted Darunavir (DRV/co) and Emtricitabine/Tenofovir Alafenamide (F/TAF) in Children With HIV

The goal of this clinical study is to learn more about the safety and dosing of study drugs, cobicistat-boosted Atazanavir (ATV/co), cobicistat-boosted darunavir (DRV/co) and emtricitabine/tenofovir alafenamide (F/TAF), in children (age ≥ 4 weeks to \< 18 years) with HIV.

Key Inclusion Criteria: * HIV-1 infected, virologically suppressed males and females age ≥ 4 weeks to \< 18 years (according to requirements of enrolling Cohort). * Body weight at screening ≥ 25 to \< 40 kg (Cohort 2); ≥ 14 to \< 25 kg (Cohort 3); ≥ 3 to \< 25 kg (Cohort 4); ≥ 3 to \< 14 kg (Cohort 5). * Stable antiretroviral (ARV) regimen for a minimum of 3 months prior to the screening visit. * Participants enrolled prior to implementation of Amendment 7: 2 nucleoside reverse transcriptase inhibitors (NRTIs) and ritonavir-boosted atazanavir (ATV/r) once daily or ritonavir-boosted darunavir (DRV/r) once daily or twice daily. * Participants enrolled after the implementation of Amendment 9: * Cohorts 2, 3 and 4 (Group 1): 2 NRTIs plus a third agent per local prescribing guidelines. Participants will switch from their current third agent to ATV or darunavir (DRV) at Day 1. Participants taking DRV must be on once-daily dosing or must switch to once daily at or prior to Day 1. Cohort 4 (Group 1), participants may also switch their current third agent to lopinavir boosted with ritonavir (LPV/r) at Day 1. Participants will switch their NRTI backbone to emtricitabine/tenofovir alafenamide (coformulated; Descovy®) (F/TAF). * Cohort 4 (Groups 2 to 4) and Cohort 5 (Groups 1 to 3): 2 NRTIs plus a third agent per local prescribing guidelines or treatment naive. Participants on treatment will switch from their current third agent to ATV or LPV/r (Cohort 4 (Groups 2 to 4)), or to a third unboosted agent (Cohort 5 (Groups 1 to 3)). Participants will switch their NRTI backbone to F/TAF. * Participants undergoing dose modifications to their ARV regimen for growth or switching medication formulations are considered to be on a stable ARV regimen. * Documented plasma human immunodeficiency virus type 1 (HIV-1) ribonucleic acid (RNA) for ≥ 3 months preceding the screening visit: * Participants enrolled after the implementation of Amendment 9: * For Cohorts 2, 3, and 4 (Group 1), virologically suppressed ≥ 3 months preceding the screening visit: HIV-1 RNA \< 50 copies/mL on a stable regimen (or undetectable HIV-1 RNA level according to the local assay being used if the limit of detection is ≥ 50 copies/mL). * For Cohorts 4 (Groups 2 to 4) and Cohort 5 (Groups 1 to 3), on an ARV regimen irrespective of plasma HIV-1 RNA copies or treatment naive; a participant is considered treatment naive, if ARVs were given for prevention of mother-to-child transmission but not for HIV treatment. * For virologically suppressed participants, unconfirmed virologic elevations of HIV-1 RNA ≥ 50 copies/mL (transient detectable viremia, or "blip") prior to screening are acceptable. If the lower limit of detection of the local HIV-1 RNA assay is \< 50 copies/mL (eg, \< 20 copies/mL), the plasma HIV-1 RNA level cannot exceed 50 copies/mL on 2 consecutive HIV-1 RNA tests. * Adequate renal function: Estimated glomerular filtration rate (eGFR) ≥ 90 mL/min/1.73m2 using the Schwartz formula. If ≥ 1 year old, eGFR greater than or equal to the minimum normal value for age using the Schwartz formula. If \< 1 year old as follows: * Age minimum value for eGFR (mL/min/1.73 m2) \> 28 days to ≤ 95 days is 30, ≥ 96 days to ≤ 6 months is 39, \> 6 to \< 12 months is 49. * Participants must not have documented or suspected resistance to applicable study drugs including emtricitabine (Emtriva®) (FTC), TFV, ATV, DRV, or LPV. Participants \< 14 kg (Cohorts 4 (Groups 2 to 4) and 5 (Groups 1 to 3)) with M184V/I AND HIV-1 RNA \< 50 copies/mL will be allowed. * Positive confirmatory HIV test (confirmatory nucleic acid-based testing if \< 18 months of age). * Cohort 4 (Groups 2 to 4) and Cohort 5 (Groups 1 to 3): Last dose of nevirapine or efavirenz, if applicable, ≥ 14 days prior to enrollment. Note: Other protocol defined Inclusion/Exclusion criteria do apply.