text: Investigating the association between mass transit adoption and COVID 19 infections in US metropolitan areas Urbanization introduces the threat of increased epidemic disease transmission resulting from crowding on mass transit The coronavirus disease 2019 COVID 19 pandemic which has directly led to over 600 000 deaths in the US as of July 2021 triggered mass social distancing policies to be enacted as a key deterrent of widespread infections Social distancing can be challenging in confined spaces required for transportation such as mass transit systems Little is published regarding the degree to which mass transit system adoption effects impacted the rise of the COVID 19 pandemic in urban centers Taking an ecological approach where areal data are the unit of observation this national scale study aims to measure the association between the adoption of mass transit and COVID 19 spread through confirmed cases in US metropolitan areas National survey based transit adoption measures are entered in a negative binomial regression model to evaluate differences between areas The model results demonstrate that mass transit adoption in US metropolitan areas was associated with the magnitude of outbreaks Higher incidence of COVID 19 early in the pandemic was associated with survey results conveying higher transit use Increasing weekly bus transit usage in metropolitan statistical areas by one scaled unit was associated with a 1 38 95 CI 1 25 1 90 times increase in incidence rate of COVID 19 a 10 increase in weekly train transit usage was associated with an increase in incidence rate of 1 54 95 CI 1 42 2 07 times These conclusions should inform early action practices in urban centers with busy transit systems in the event of future infectious disease outbreaks Deeper understanding of these observed associations may also benefit modeling efforts by allowing researchers to include mathematical adjustments or better explain caveats to results when communicating with decision makers and the public in the crucial early stages of an epidemic 