PROJECT SUMMARY / ABSTRACT Background: Transactional sex may account for a significant number of new HIV infections in sub-Saharan Africa, yet most studies have focused on female partners in these relationships. Male partners are an important group to study as they may drive demand for transactional sex and their HIV risk is unclear. To design targeted interventions for effective prevention of HIV in heterosexual partnerships, there is an urgent need to understand the risk of HIV transmission and acquisition among male transactional sex partners. Study Goal and Aims: This study will characterize male engagement in heterosexual transactional sex partnerships and examine the causal relationship between transactional sex and HIV in Rakai, Uganda. Specific aims are to: 1) identify factors associated with transactional sex among men; 2) estimate the causal effect of transactional on incident HIV among men; 3) measure perceptions and response processes of an RCCS transactional sex measure through cognitive interviewing; and 4) qualitatively explore the context for transactional sex and causal mechanisms that may heighten risk for HIV in transactional sex partnerships. Approach: This explanatory, sequential mixed-methods study will leverage the data and infrastructure of Rakai Community Cohort Study (RCCS), an open, population-based cohort study of approximately 18,000 individuals in 50 communities in Rakai District, Uganda. Aims 1 & 2 will be a secondary analysis of men aged 15-49 from 3 rounds of RCCS data collected between 2015 and 2019. Aim 1 will use multivariable log- binomial regression to examine factors associated with transactional sex. Aim 2 will use marginal structural models to quantify the causal effect of transactional sex on incident HIV, controlling for time-dependent confounding. Aims 3 & 4 entail primary data collection of semi-structured in-depth interviews of men ages 18-49 reporting transactional sex. Aim 3 will examine how men perceive and respond to transactional sex questions in the RCCS. Aim 4 will conduct 25-40 semi-structured in-depth interviews with men to explore the contextual factors motivating transactional sex and the potential causal mechanisms that heighten risk for HIV in transactional sex partnerships. Fellowship Information: The proposed research will serve as the doctoral dissertation of Ms. Holly Nishimura. The training is guided by one Sponsor, two Co-sponsors, and one Scientific Advisor who offer complementary methodological and topical expertise in HIV epidemiology, mixed methods research and masculinity theory. Training includes coursework, field research in Uganda (pending travel COVID-related travel restrictions), and other activities to prepare Ms. Nishimura to become a leading independent researcher in global HIV prevention and control. The proposed study directly aligns with NIH research priorities to reduce gendered disparities in HIV incidence through cross-cutting applications in epidemiologic methods...