Evaluating the impact of achieving cascade equality in Eswatini: a modelling study on the prevention impacts of antiretroviral therapy

Objective: Inequalities in the antiretroviral therapy (ART) cascade across subpopulations remain an ongoing challenge in the global HIV response. Eswatini achieved the UNAIDS 95-95-95 ART cascade targets by 2020, with differentiated programs to minimize inequalities across subpopulations, including for female sex workers (FSW) and their clients. We sought to estimate the impacts of this achievement, through a retrospective impact evaluation of ART scale-up in Eswatini. Design: Drawing on population-level and FSW-specific surveys, we developed a compartmental model of heterosexual HIV transmission, and calibrated it to observed HIV prevalence, incidence, and ART cascade scale-up in Eswatini. Methods: We defined four counterfactual scenarios in which the population overall reached only 80-80-90 by 2020, but where FSW, clients, both, or neither were disproportionately left behind, reaching only 60-40-80. We estimated additional HIV infections by 2020 in counterfactual vs observed scenarios, and identified epidemic conditions which maximized differences. Results: Compared with observed cascade scale-up in Eswatini, leaving behind neither FSW nor their clients led to median (95% CI) 8.8 (6.3, 10.9)% additional infections by 2020 vs 14.3 (10.8, 18.6)% if both were left behind — a 63 (31, 128)% increase. The impact of leaving behind FSW and/or clients was largely determined by their population sizes and HIV incidence ratio among clients vs men overall. Conclusions: Inequalities in the ART cascade across subpopulations can undermine the anticipated prevention impacts of cascade scale-up. As Eswatini has shown, addressing inequalities in the ART cascade that intersect with transmission risk can maximize incidence reductions from cascade scale-up.