PROJECT SUMMARY / ABSTRACT Significance: Female sex workers (FSW) living with HIV are disproportionately marginalized and face a myriad of barriers to sustained engagement in HIV care and viral suppression. In 2022, South Africa remains the epicenter of HIV globally, and among FSW in the country, diverse experiences, vulnerabilities, and treatment outcomes exist. Tailored, multifactorial implementation strategies to support HIV care and viral suppression are needed for FSW whose needs are not being met, yet, it is not feasible nor effective to offer everything to everyone. Study Goal and Specific Aims: This study will provide a detailed understanding of when FSW engage with implementation strategies to support HIV care and treatment, who engages, as well as how implementation determinants influence outcomes, illuminating potential mechanisms for optimizing HIV treatment support strategies. Specific Aims are to: 1) Identify patterns of engagement among FSW living with HIV and determine their correlates and association with retention and viral suppression over time; 2) Explore how fidelity of strategy implementation (i.e., strategy dose administered) impacts the relationship between strategy exposure and clinical outcomes among FSW living with HIV; and 3) Characterize contextual factors influencing strategy implementation and the effect on retention and viral suppression among FSW living with HIV. Approach: This study will leverage existing quantitative, qualitative, and implementation data and infrastructure of the Siyaphambili trial. The Siyaphambili study employed a sequential multiple assignment randomized trial to test two implementation strategies among a cohort of 777 non-virally suppressed FSW living with HIV over an 18-month follow-up period. The proposed study utilizes group-based trajectory modeling to identify distinct trajectories of longitudinal FSW engagement (Aim 1), a component path analysis to explore the role of programmatic implementation fidelity (Aim 2), and a moderator analysis to characterize implementation determinants, interpreted and contextualized through in-depth interviews (Aim 3). All proposed Aims represent novel analyses of existing, cleaned data collected from the Siyaphambili study. Fellowship information: The proposed research is the doctoral dissertation of Ms. Carly Comins. The training plan consists of selected coursework, tailored ongoing mentorship, and professional development to foster the successful completion of the proposed research and to prepare Ms. Comins to become an independent HIV epidemiologist and implementation scientist supporting HIV-related implementation research for key populations. The proposed study directly aligns with the NIMH’s goals and the priorities of the Division of AIDS Research, including leveraging pragmatic effectiveness-implementation research to enhance understanding of the real-world impact of evidence-based interventions; improving methods to match interventions to margin...