PROJECT SUMMARY / ABSTRACT Engaging the most marginalized people with HIV (PWH) in treatment is crucial to end the HIV epidemic in the U.S. The Ending the HIV Epidemic (EHE) strategy in King County, Washington is designed to improve HIV services for people with complex barriers to care, including homelessness or unstable housing, severe mental illness, and substance use disorders. The health department in King County has developed a robust information exchange system with data from HIV surveillance, jail booking, and emergency room visits to guide public health interventions to re-engage PWH in care (“Data to Care”). The overall goals of the proposed project are to use data science to improve public health Data to Care activities and to establish a community-engaged partnership team that will collaboratively develop a future data-guided mobile outreach & treatment team for high-need PWH. For Aim 1, we will develop and validate an algorithm to prioritize individual PWH for interventions to improve engagement in HIV care and behavioral health services. Using a machine learning method (classification and regression tree models), we will analyze data from >14 years of case investigations and the current health department information exchange system. During the project year, we will newly integrate behavioral health and housing data into the information exchange. Throughout this work, we will incorporate input from our community partners on our use of data for care re-engagement. For Aim 2, we will establish a community-engaged partnership team to develop a mobile outreach & treatment intervention for high-need PWH in King County. This team will include representatives of key service agencies and the health department, people with lived experience, and university researchers. We will use the Transcreation Framework for community-engaged implementation science research to develop a mobile outreach and engagement team based on the Assertive Community Treatment approach, an evidence-based service delivery model that uses a multidisciplinary team to deliver community-based mental health treatment to individuals with severe mental illness. The expected outcomes of this project include: 1) a validated case prioritization algorithm for HIV care re-engagement interventions that will immediately inform health department Data to Care efforts and guide a future mobile outreach & treatment team; 2) a solidified partnership of community agencies, community members with lived experience, the health department, and researchers, which will collaboratively implement and evaluate the future mobile outreach & treatment team; and 3) a refined intervention, implementation strategy, and implementation research logic model to be used in a grant proposal to study the data-guided mobile outreach & treatment team.