How does religiosity affect a state’s adoption of novel, COVID-19 vaccines? We hypothesize that religiosity (the degree to which people in a society are highly religious) will have a deterring effect on the adoption of mRNA COVID-19 vaccines. We examine this idea using U.S. state-level data from the Centers for Disease Control in February - March 2021 on the initial adoption of mRNA COVID-19 vaccines (i.e., Pfizer-BioNTech and Moderna vaccines) during the COVID-19 crisis. Post hoc analysis reveals that this negative effect is broadly consistent across different Protestant groups (i.e., Evangelical Protestants, Historically Black Protestants and Mainline Protestants) while Catholics show mostly no significant effect on adoption, except for a marginal effect in February 2021. The findings demonstrate the importance of religious institutions in the determination of a state’s rate of adoption of mRNA COVID-19 vaccines.