- Antiretroviral (ARV) treatment-naive or ARV treatment-experienced. Treatment-experienced participants are defined by previous exposure to ARVs through either prior treatment for HIV infection or through postnatal treatment with ≥1 ARV for the prevention of mother to child transmission. For the purposes of this study, participants exposed to ARVs in utero or intrapartum may be included in the study but will be considered treatment naive. ATV-naive participants must have genotypic sensitivity at screening to ATV (fold change in susceptibility <2.2) and to both components of the local NRTI backbone. The NRTIs must have been approved for pediatric use at the local country level.