Project Summary Frequent HIV testing among persons at increased HIV risk is crucial for achieving early diagnosis and prompt antiretroviral therapy initiation – key steps to reducing morbidity and eliminating transmission. HIV retesting can also support ongoing efforts to scale-up pre-exposure prophylaxis. Despite guidelines recommending frequent retesting among adults at higher risk, studies from sub-Saharan Africa suggest that retesting occurs infrequently. Behavioral economics provides insights into why people may not engage in behaviors like HIV retesting. Low HIV retesting stems from structural factors (e.g. stigma and opportunity costs), as well as various biases and heuristics in people’s beliefs and behavior, constraints in attention, and reliance on incorrect mental models. Behavioral economics has also identified low-cost interventions (“nudges”) that effectively account for these features of behavior. Building on our prior work that has identified barriers to HIV retesting, we have applied a Behavioral Design framework to select nudges that are effective in addressing the psychological underpinnings of each barrier to retesting. We will adapt the nudges to HIV retesting and local contexts through a participatory prototyping approach that brings together scientists and end users. We will then conduct a large-scale, multi- arm field experiment that features individual randomization and tests many different nudges. This “mega-trial” approach has recently been used to test behavioral interventions to promote other health behaviors in the US and offers many advantages over standard experiments that only test 1 or 2 interventions. With the same outcome being examined in all study arms, mega-trials allow for direct comparison of different interventions. Unlike 2-arm experiments, they also make it possible to identify which interventions work best for whom. Leveraging the infrastructure of the SAPPHIRE trial of novel HIV biomedical prevention and treatment approaches in Kenya and Uganda, we will enroll a large study population of persons at high risk of HIV to conduct a mega-trial of low-cost behavioral interventions to promote HIV retesting. The project will have the following aims: AIM 1. Adapt behavioral economics interventions to promote HIV retesting using a participatory prototyping approach in rural Kenya and Uganda. AIM 2. Determine the effectiveness of many behavioral economics interventions to promote HIV retesting among high-risk adults in a multi-arm randomized trial. AIM 3. Estimate heterogeneous treatment effects and identify optimal demand creation interventions. The research we propose can accelerate the pace of discovery of interventions to increase HIV retesting and transform future approaches to designing, testing and comparing interventions for health behaviors.