INTRODUCTION: Applied psychologists have long relied on surveys and experiments to study intentions, but these are often slow, costly, and potentially biased. Online search data promises a cheap, always-on, and accurate alternative: Could we know what people are thinking without asking them? Could we predict their behavior, just by knowing what they search for? Using the COVID-19 pandemic as a case study, we find that Google searches for "vaccine" strongly predicted actual U.S. vaccination rates, while state-level differences in specific search terms correlated with differences in vaccine uptake. PURPOSE: This study uses COVID-19 vaccination to test how digital traces reveal motivational dynamics, offering psychologists a new tool for anticipating health-seeking behavior. METHOD: We analyzed weekly U.S. Google search volumes at the national and state levels for "vaccine" alongside CDC vaccination data (Dec 2020-Jun 2022). Study 1 asks: Do search patterns predict vaccination? Study 2 asks: Is online search a weaker indicator of vaccination intention in states with lower vaccine uptake? We estimated correlations, geographic models, and time-series models, considering lags and stationarity concerns. RESULTS: Study 1 finds: Online search for "vaccine" predicted vaccination with near-perfect correlation (rho= 0.97, when search volume was lagged by 3 weeks) in the early rollout phase. Study 2 finds: Online search meant different things across states, signaling vaccine readiness in high-vaccinating states but highlighting side-effect concerns in lower-vaccinating states. CONCLUSIONS: Online search can serve as a powerful proxy for intention formation, predicting behavior with remarkable accuracy in specific contexts. For applied psychology, these findings show how Big Data can complement surveys and experiments by capturing cognitive, motivational, and societal processes in real-world settings. This approach is especially valuable for health psychology, societal development, and applied cognitive psychology, offering practical insights for interventions and reducing disparities in health behavior.