DescriptionCOVID-19 pandemic places unprecedented strain on the US healthcare system; primary care is no exception. Primary care serves as the lynchpin of the US healthcare system, treating patients with both acute and chronic illnesses amidst the pandemic. In the last decade, primary care services have shifted toward a team-based approach for delivering care. Drawing from the science of effective teams, the current study examines the extent to which adaptation within primary care teams occurs in the presence of an adaptive stimulus (i.e., COVID-19), exploring evidence-based factors from the science of effective teams which may impact their success. To do this, our study leverages a large archival study of Patient Aligned Care Teams (PACTs) nationwide– the Veterans Health Administration’s adaptation of the Patient Centered Medical Home model- alongside Pandemic Vulnerability Index (PVI) data to examine the direct and indirect impacts of COVID-19 on primary care team configurations, turnover, coordination, adaptive performance, and patient-related outcomes. Bayesian approaches, using 2 Markov Chain Monte Carlo chains for each parameter, reveal multilevel effects of COVID-19 on health systems and teamwork nationwide. More specifically, our results reveal no direct impact of COVID-19 as an adaptive stimulus on adaptive team performance (B = -.003) . Further, fluidity of team members (turnover) within a team failed to explain the relationships between stimulus intensity and team coordination (B = .26); however, team coordination positively predicts adaptive team performance (B = .00). Interestingly, countermeasure data (i.e., public health interventions such as social distancing and intervention testing) present at facility locations moderated the relationship between adaptive stimulus intensity and team member fluidity (B= 1.49), such that greater turnover was seen in areas with strong stimulus intensity and low implementation of public health countermeasures. Adaptive team performance did not significantly predict clinical or patient-related outcomes (B= .76). Despite this, results from our study attest to the need to preserve team coordination in the presence of strong adaptive stimulus effects and implement targeted countermeasures to ease team workload and burden to optimize well-being. Teamwork can act as a protective factor against high levels of workload, burnout, and turnover; however, this is the first known empiric study testing the extent to which theories of team adaptation withstand in an extreme adaptive stimulus in an applied setting, such as COVID-19 in primary care.