PROJECT SUMMARY/ABSTRACT Adolescent alcohol abuse conveys significant disease burden. The coronavirus disease 2019 (COVID-19) pandemic has wrought unprecedented changes to adolescents’ daily lives, affecting several domains with well- establish links to alcohol use (e.g., increased stress and family conflict). However, there is no published data the course of adolescent drinking across the duration of the pandemic; the specific effects of each pandemic-related change (e.g., social distancing, remote schooling, parents working from home) on adolescents’ drinking; risk/resiliency factors exacerbating or buffering against pandemic’s impact on drinking; or the relation of COVID- 19 infection/vaccination among youth and family members to pre-morbid or post-morbid drinking. Thus, we lack the time-sensitive information necessary to guide an effective public health strategy. We propose to address this urgent, time-sensitive need using secondary data analysis of the Adolescent Brain and Cognitive DevelopmentSM Study, a large (N=11,880), longitudinal, nationwide cohort of adolescents spanning 21 study sites across the United States. Participants had been followed prospectively for a median of 29 months before the beginning of the pandemic and were well-characterized across multiple assessments of alcohol and drug use, mental/physical health, and family/community environment. 9,031+ participants and their parents completed up to seven waves of surveys between May 2020 and May 2021, each measuring teen’s drinking and experiences during the COVID-19 pandemic. By combining a well-characterized, sociodemographically diverse, nationwide cohort recruited before the pandemic with serial assessments of that cohort across the first 14 months of the pandemic, the ABCD Study® has outstanding potential to study adolescent alcohol use during the pandemic. We pursue four specific aims. Aim 1: Describe adolescent drinking over the course of the COVID-19 pandemic and evaluate associations of drinking with (a) engagement in public health precautions (e.g., social distancing), (b) changes in schooling status and format, (c) disruptions to daily routines, (d) emotional health and stress/worry about COVID-19, (e) family stress/discord, and (f) parents’ drinking. Aim 2: Identify vulnerable groups of adolescents whose alcohol use has been impacted most severely by the COVID-19 pandemic to inform targeted screening and support, testing for differences by youth sex/gender/age/race/ethnicity, socioeconomic disadvantage, family and peer environment, or youth predisposition to early drinking. Aim 3: Evaluate the association of COVID-19 infection in adolescents and their family members with adolescents’ pre-morbid and post-morbid drinking. Aim 4: Determine the impact of youth vaccination on subsequent alcohol use patterns. Research activities are structured to produce findings rapidly and disseminate them widely, to best support the time-sensitive public health strategy.