COVID-19 vaccine coverage in the US has marked demographic and geographical disparities, but few explanations exist for them. Our paper aimed to identify behavioral and social drivers that explain these vaccination disparities. Participants were a national sample of 3562 American adults, recruited from the Ipsos KnowledgePanel. Participants completed an online survey in spring 2021, when COVD-19 vaccination was available for higher-risk groups but not yet available to all US adults. The survey assessed COVID-19 vaccination stage (intentions and vaccine uptake), constructs from the Increasing Vaccination Model (IVM) domains (thinking and feeling, social processes, and direct behavior change), self-reported exposure to COVID-19 vaccine information, and demographic characteristics. Analyses used multiple imputation to address item nonresponse and linear regressions to conduct mediation analyses. Higher COVID-19 vaccination stage was strongly associated with older age, liberal political ideology, and higher income in adjusted analyses (all p < .001). Vaccination stage was more modestly associated with urbanicity, white race, and Hispanic ethnicity (all p < .05). Some key mediators that explained more than one-third of demographic differences in vaccination stage were perceived vaccine effectiveness, social norms, and recommendations from family and friends across most demographic characteristics (all p < .05). Other mediators included safety concerns, trust, altruism, provider recommendation, and information seeking. Access to vaccination, barriers to vaccination, and self-efficacy explained few demographic differences. One of the most reliable explanations for demographic differences in COVID-19 vaccination stage is social processes, including social norms, recommendations, and altruism. Interventions to promote COVID-19 vaccination should address social processes and other domains in the IVM.