Project Summary/Abstract Existing HIV care systems in the United States, usually based on scheduled appointments, are often inadequate for people living with HIV who have significant psychosocial and structural barriers to engagement in care (i.e. homelessness/unstable housing, substance use disorders, severe mental illness). Rather than connect these individuals to an incompatible system of HIV care, new approaches are needed that reduce barriers to care engagement and offer increased flexibility. In this study, we seek to implement an evidence- informed multicomponent clinical intervention that includes drop-in (i.e. no appointments) multidisciplinary HIV primary care, mobile HIV care, staged escalation/de-escalation of care intensity as needed, and active referral of patients from community-based clinical and non-clinical sites into this care model. This clinical intervention will be implemented at four diverse care sites in San Francisco and Alameda counties, both priority jurisdictions in the U.S. Ending the HIV Epidemic (EHE) strategy: an academic safety net HIV clinic, a needle exchange program, and two federally qualified health centers. Eligibility criteria include: 1) current HIV viral load ≥200 copies/mL or off antiretroviral therapy, 2) history of poor HIV care engagement, and 3) homelessness/ unstable housing, any mental health disorder, or any illicit substance. We use the Consolidated Framework for Implementation Research (CFIR) and RE-AIM implementation frameworks to guide implementation strategy selection and our implementation and clinical effectiveness evaluation. In Aim 1, we will use implementation mapping to assess barriers and facilitators of implementation and convene key stakeholders to contextually integrate the clinical intervention and finalize the implementation strategies. In Aim 2, we will conduct a hybrid type 2 implementation-effectiveness study to evaluate the effect of clinical intervention implementation on co-primary outcomes of Reach (any HIV primary care visit) and Effectiveness (any HIV viral load <200 copies/mL) among patients referred to the care model over 12 months of follow-up (n=400), comparing outcomes to two propensity score matched control groups (400 contemporaneous controls identified using Department of Public Health data and 400 historical controls identified at study sites). We will also assess clinic-level implementation outcomes. In Aim 3, we will evaluate and model the individual, clinic, and population-level impacts of the intervention approach using heterogeneity and health equity analysis, cost/ cost-effectiveness analysis, scenario modeling of optimal and reduced component scenarios and population- level impact. Our multidisciplinary study team has a strong track record of implementation research to improve HIV care engagement among vulnerable populations. The proposed study will provide robust evidence for a drop-in/mobile HIV care approach and strategies to support implementation at a...