Western Kenya has among the world’s highest prevalence of HIV, with one in four adults infected in Siaya and Homa Bay Counties. Longitudinal surveillance of a community in Siaya found that incidence has fallen by two-fold between 2011 and 2016. We used mathematical modeling to estimate the relative contribution of antiretroviral therapy (ART) and voluntary male medical circumcision (VMMC) to the declines in HIV incidence in the Western Kenya region, including the county of Siaya.
EMOD-HIV, an individual-based HIV transmission and care continuum model, was used to simulate the HIV pandemic in Western Kenya. The model was calibrated to age-, sex-, and county-specific HIV prevalence estimates from four national surveys, as well as estimates of population size and structure, number on ART, number receiving VMMC, and national targets for VMMC coverage. Conservatively, we assumed a sustained ART coverage of approximately 60%. Calibration yielded 250 best-fitting model trajectories for each of six counties comprising the Nyanza region of Western Kenya. In Siaya County, EMOD-HIV recapitulated the halving of HIV incidence over 2011-2016 at the county level, despite the model fitting process not directly utilizing incidence estimates from the longitudinal surveillance site in this county. The baseline model trajectories were modified to simulate what would have happened in the absence of ART and/or VMMC.
Estimated HIV incidence declined drastically in Siaya due to scale-up of ART and VMMC, without which incidence would have remained stable at 1.7 new infections per 100 person-years among adults age 15-49 (Figure 1). Incidence peaked in 2002, fell to half of its peak by 2018, and continued to decline to one-third of peak levels by 2028. ART is the predominant cause of incidence declines up until 2025, after which VMMC is expected to surpass ART as a driver of incidence decline, provided Siaya achieves and maintains a target of 80% VMMC coverage. Similar trends were found in other high-prevalence counties in Western Kenya.
Epidemiological modeling suggests that observed incidence declines in Siaya County, Western Kenya, can be fully attributed to scale-up of ART and VMMC, without which incidence would have remained stable. Incidence is expected to continue to decline due to these interventions, but enhanced efforts to prevent HIV infections will be required to accelerate declines and bring incidence to low levels.