Abstract
<jats:title>Summary</jats:title> <jats:sec> <jats:title>Background</jats:title> <jats:p>As HIV incidence declines in African settings with high treatment coverage, it remains unclear how transmission is structured within populations and whether new infections arise from external introductions or local transmission. We characterized the molecular epidemiology of ongoing transmission in a mature multi-subtype epidemic in Uganda.</jats:p> </jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p>We analyzed HIV genome sequences and survey data from the Rakai Community Cohort Study collected between 1994 and 2019. We identified phylogenetic clusters at 5·3% and 2·5% genetic distance thresholds and inferred long-horizon transmission chains with phylogeographic models. Newly diagnosed infections identified between 2016 and 2019 were mapped onto subtype- specific phylogenies to assess their origins and transmission context. A Bayesian negative binomial branching process model estimated undersampled chain sizes and case reproduction numbers.</jats:p> </jats:sec> <jats:sec> <jats:title>Findings</jats:title> <jats:p>Among 4215 participants living with HIV between December 2016 and May 2019, 474 were newly diagnosed, of whom 269 had at least one pure-subtype sequence available. We identified 649 phylogenetic clusters at 5·3% genetic distance and 673 phylogeographic chains including ≥2 individuals. Most clusters and chains were small (median sizes 2 [IQR 2–3] and 3 [2–4], respectively), with new diagnoses rarely clustered together. Only 46/269 (17·1%) new diagnoses had phylogeographic external origins, while the remaining 82·9% were partially or fully linked to local chains. Mixed-subtypes/recombinant chains were larger and had higher case reproduction numbers (A1/D: 0.84 [95% CrI: 0.79-0.93]; mixed: 0.84 [0.73-0.97]) than single- subtype chains (A1: 0.56 [0.51-0.60]; D: 0.63 [0.59-0.66]; C: 0.55 [0.41-0.71]), yet all estimates were less than one.</jats:p> </jats:sec> <jats:sec> <jats:title>Interpretation</jats:title> <jats:p>HIV transmission was fragmented across numerous, slowly propagating lineages, maintained by local clusters with occasional introduction. Continued transmission across many chains suggests that further reductions in HIV incidence will require maintaining high levels of population-wide treatment and prevention coverage.</jats:p> </jats:sec> <jats:sec> <jats:title>Funding</jats:title> <jats:p>The National Institute of Allergy and Infectious Diseases, the Gates Foundation, and the HIV Prevention Trials Network Laboratory Center</jats:p> </jats:sec>