Vaccines are highly effective for curbing the spread of SARS-CoV-2 (COVID-19). Yet, millions of Americans remain hesitant about getting vaccinated, jeopardizing our ability to end the COVID-19 pandemic by fueling the spread and development of new variants. We show that brief video-based messages of encouragement addressing specific COVID-19 vaccine concerns increase vaccination intentions, and that vaccination intentions, in turn, are predictive of future vaccine uptake. Results from our online experiment reveal that willingness to get vaccinated is driven by messages that increase confidence in COVID-19 vaccines and perceived behavioral control to get vaccinated. Importantly, messages were particularly effective among more skeptical populations including people who identify as politically conservative or moderate and those who express low trust in government institutions. Our findings corroborate the real-world behavioral significance of vaccination intentions, and devise how even short, scalable online messages can provide governments and health authorities an inexpensive, yet effective tool for increasing intentions to vaccinate against COVID-19 among populations most reluctant to get them.