This study describes drug resistance among infants whose mothers were enrolled in VESTED, an international open-label trial.  ART naïve adults 14-28 weeks gestation (participants were allowed to have received ≤ 14 days of ART to avoid delays in therapy initiation) were randomized to DTG + FTC/TDF, DTG + FTC/TAF, or EFV/TDF/FTC and followed through 50 weeks postpartum.  Overall, participants were randomized to study ART at a median 21.9 wks GA and 78% breastfed.  Virologic failure was detected in 42, and transmission was detected in 4 (out of a total 617 liveborn infants with complete follow-up).  Case A involved likely in utero infection from pre-treatment viremia, as virologic suppression was achieved and maintained through delivery after study ART initiation (both baseline maternal and infant GRT indicated wild-type virus).  Case B involved likely peripartum transmission due to ongoing viremia throughout pregnancy; further, the evolution of NNRTI mutations [based on results of serial infant testing over the first 2 weeks of life] suggested resistance selected by infant NVP prophylaxis.  Case C involved likely transmission during breastfeeding with new-onset postpartum viremia; although this infant also received NVP prophylaxis, pre-study EFV use (7 days) with emergent K103N in maternal GRT, and K103N detected among 25/25 infant templates sequenced at initial diagnosis suggests transmission of drug resistant virus.  Case D involved persistent viremia on EFV-based therapy, including during breastfeeding (range 59-323 copies/mL), possibly leading to late postnatal transmission.  Authors note NRTI and NNRTI DRM identification in infant samples may be related to subinhibitory concentrations of maternal ARVs in breastmilk selecting for resistance.