It is puzzling that a sizeable percentage of people refuse to get vaccinated against COVID-19. This study aimed to examine social psychological factors influencing their vaccine hesitancy. This longitudinal study traced a cohort of 2663 individuals in 25 countries from the time before COVID-19 vaccines became available (March 2020) to July 2021, when vaccination was widely available. Multilevel logistic regressions were used to examine determinants of actual COVID-19 vaccination behavior by July 2021, with country-level intercept as random effect. Of the 2663 participants, 2186 (82.1%) had been vaccinated by July 2021. Participants' attitude toward COVID-19 vaccines was the strongest predictor of both vaccination intention and subsequent vaccination behavior (p < .001). Perceived risk of getting infected and perceived personal disturbance of infection were also associated with higher likelihood of getting vaccinated (p < .001). However, religiosity, right-wing political orientation, conspiracy beliefs, and low trust in government regarding COVID-19 were negative predictors of vaccination intention and behavior (p < .05). Our findings highlight the importance of attitude toward COVID-19 vaccines and also suggest that certain life-long held convictions that predate the pandemic make people distrustful of their government and likely to accept conspiracy beliefs and therefore less likely to adopt the vaccination behavior.