ABSTRACT COVID-19 has created an unparalleled health crisis across the globe that has ravaged underserved and vulnerable populations in the US, including in the territories of Puerto Rico (PR) and the U.S. Virgin Islands (USVI). While testing for COVID-19 has increased in the Caribbean territories, it is still deeply inadequate. The USVI and PR have experienced barriers to wide-spread testing, including an overall shortage of testing equipment and supplies, which has led to a sharp increase in cases since early July. Data from our Eastern Caribbean Health Outcomes Research Network Cohort Study (ECS) indicate nearly 61% of our population-based random sample (n=3000) has at least one chronic condition, making them high risk for severe complications from COVID-19. We propose to enhance and amplify the COVID-19 diagnostic testing cascade in the USVI and PR by utilizing community-based assets— Federally Qualified Health Centers (FQHCs) and their community organization (CBO) partners—to address barriers to full participation in the testing continuum from diagnosis through to self-isolation and quarantine. We will partner with 3 FQHCs—one in PR and two in USVI—and CBO partners for each. We will provide FQHCs with protocols for diagnosis and follow-up and will procure new testing technologies to make testing more efficient. CBOs connected to the FQHCs will help reach underserved community members not currently being tested, such as those with elevated medical risk, those living without homes, or those living in congregate settings such as nursing homes. After notification of a positive result, we will support FQHCs to provide ambulatory care for active clinical monitoring at home and will deploy a screener for social determinants of health. CBO partners will connect patients with social needs to resources to support quarantine and self-isolation recommendations. We will additionally assess the diffusion and uptake of FDA authorized and approved testing technologies across existing and emerging testing models. We will utilize a rigorous quasi-experimental approach to focus on assessing our implementation outcomes. Specifically, we will employ a hybrid III implementation-effectiveness study design. The primary implementation outcome will be reach—a measure of the amount of testing that is being done in the community— using an interrupted time series design (ITS). Other outcomes of interest include adoption of new testing technologies, fidelity to the intervention, its acceptability, and its effectiveness. We will deploy the RE-AIM framework to guide the evaluation of the intervention. Throughout the project period, we will collaborate with the RADx-Up Coordination and Data Collection Center (CDCC) across all activities, including harmonizing data collection and procuring new COVID-19 testing technologies. We will also work closely with the Community Engagement domain to enhance the experience of our participating stakeholder partners. Finally, the enhanced...