ABSTRACT Youth living with HIV (YLH) have high rates of viral non-suppression and non-retention, particularly during the period when they transition from adolescent to adult autonomous care. In partnership with the Kenya Ministry of Health (MoH), we used a community engagement approach to develop the Adolescent Transition Package (ATP), a healthcare worker (HCW) toolkit that includes structured educational materials and tracking tools to facilitate the transition process. We recently tested the ATP in a cluster randomized clinical trial (cRCT) of >1000 YLH in 20 clinics and found the ATP effective at improving transition readiness. The ATP was acceptable and feasible. The Kenya MoH is committed to supporting transition among YLH and scaling up the ATP. We propose testing strategies to SCALE-UP the ATP in Kenya using a data driven youth led implementation strategy. Aim 1: Using a hybrid III implementation-effectiveness design, compare two implementation strategies for scale- up of ATP: Kenya MoH SoC implementation strategy (cascading training of trainers) to an enhanced scale-up implementation strategy (ATP-YES) (SoC plus youth-led data- driven intervention adaptations). Approach: Thirty-two HIV clinics (8 per county) will be randomized 1:1 to SoC or ATP-YES. In intervention sites, youth leaders will lead routine ATP implementation assessments and targeted adaptations to optimize reach, effectiveness, adoption, and implementation (fidelity) through data audits, cyclical small tests of change and cross-facility learning. Aim 1a: Compare 3-year ATP reach, adoption and effectiveness between intervention and control sites. Reach: proportion of YLH age 15-24 years exposed to the ATP intervention. Adoption: proportion of HCWs using the ATP. Effectiveness: successful transfer to adult care, post-transition retention and viral suppression and transition readiness scores. Aim 1b: Using the Consolidated Framework for Implementation Research (CFIR) framework, identify determinants of ATP adoption, implementation (fidelity to the ATP) and maintenance of the ATP. Aim 1c: Summarize intervention adaptations using the Framework for Reporting Adaptations and Modifications to Evidence-based Implementation Strategies (FRAME-IS). Aim 2: To evaluate the cost-effectiveness and budget impact of the ATP-YES implementation strategy. Approach: We will conduct micro costing, HCW interviews and time and motion observations to estimate the incremental costs of implementing the ATP-YES strategy in routine HIV care. We will combine cost with effectiveness data from Aim 1a into a Markov model to project the health impact (HIV deaths and morbidity averted) and financial costs of the intervention compared to standard of care and estimate the cost-effectiveness and budget impact of ATP-YES implementation. This implementation science study focuses on sustainable implementation of an evidence-based intervention through youth-led data-driven intervention adaptations. The project align...