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.