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