Background: The AIDS Clinical Trials Group study A5353 demonstrated the efficacy and safety of dolutegravir and lamivudine for initial treatment of HIV-1 infection at week 24 in individuals with HIV-1 RNA 1000-500 000 copies/mL. Optimal ART for treatment-naive individuals must be durable. Objectives: The aim of this study was to estimate the efficacy and safety of dolutegravir plus lamivudine at week 48 and compare the efficacy in participants with baseline HIV-1 RNA ≤100 000 copies/mL versus >100 000 copies/mL. Methods: Virological success was defined as HIV-1 RNA <50 copies/mL by FDA Snapshot criteria. Definition of virological failure included confirmed HIV-1 RNA >200 copies/mL at week 24 or later. The proportion of participants with virological success was estimated using two-sided exact Clopper-Pearson 95% CI. Comparison between screening HIV-1 RNA (≤100 000 versus >100 000 copies/mL) strata was carried out by Fisher's exact test. The study was registered with ClinicalTrials.gov, number NCT02582684. Results: A total of 120 enrolled eligible participants were included in the analysis. At week 48, 102 of the 120 participants (85%; 95% CI 77%-91%) had virological success. Virological success was similar between screening HIV-1 RNA groups. Six (5%) participants had virological non-success and one additional participant experienced virological failure while on study but off study treatment. No new drug resistance mutations were observed. Six (5%) participants had study-related grade 3 or higher adverse events and none discontinued study treatment. Conclusions: These results add to the evidence that dolutegravir plus lamivudine is a safe and effective option for initial ART in individuals with HIV-1 RNA <500 000 copies/mL.