Project Summary/Abstract The COVID-19 pandemic has led to a dramatic shift to virtual treatment for patients with drug use disorder (DUD), at the same time as drug use has increased during the pandemic, and people of color are at disproportionate risk of distress and drug use. This shift raises questions about potential disparities in access to virtual treatment, given the existing ‘digital divide,’ where older patients, patients of color, and patients of lower socio-economic status may have less access to broadband, computers, and other digital devices. Patients may also face barriers such as confidentiality concerns, lack of private space for participating in virtual sessions, and dissatisfaction with the virtual therapeutic experience. At the same time, features of virtual treatment, such as convenience and elimination of transportation and its costs, may increase access and use, particularly for underrepresented patient populations. The proposed study examines disparities in virtual treatment among adult and adolescent patients with DUD in a large, diverse, health care delivery system that was able to quickly pivot to virtual DUD treatment delivery during the pandemic. Using rich electronic health record and claims data, we examine changes in DUD identification, and in several treatment measures (initiation, engagement, and retention in specialty addiction treatment; medication for opioid use disorder) comparing visit types (virtual/non-virtual, telephone, video) from a pre-COVID-19 (3/2019- 12/2019) to post-COVID-19 onset (3/2020-12/2020) time period. With large samples of 61,199 adult and 3,494 adolescent patients with DUD, we will specifically explore disparities by race/ethnicity, gender, age, and socioeconomic status. We explore differential insurance loss during this turbulent time, as well as health services utilization (e.g. emergency department, primary care, psychiatry, email secure messages) by the study population. Our rigorous analytic plan incorporates interrupted time series (ITS), and generalized estimating equation (GEE) models to address study aims. Early evidence suggests drug use, and adverse events such as opioid overdose, have increased during the COVID-19 pandemic, and virtual treatment will likely continue post-pandemic as an important treatment modality. Findings have important implications for patient care, particularly for improving access to and quality of care for vulnerable populations. Further, the study cohort will be a valuable resource for future research on virtual DUD treatment, and its long-term impact on patient outcomes.