University of California Davis What approaches are most effective at targeting vaccine misinformation on social media among users unlikely to visit fact-checking websites or engage with thorough corrections? In light of increasing levels of vaccine hesitancy in several countries, researchers are studying the role of exposure to misinformation - particularly as it spreads on social media - as one of the factors that could negatively influence vaccination attitudes and decisions. One approach being studied is the design of platform-based interventions to provide social media users with signals on content and source quality. This paper explores 2 central questions: (i) Can one such signal - fact-checking labels on misinformation - result in more favourable attitudes toward vaccines, and is the effect contingent upon race, education, and/or conspiracy ideation? (ii) Does the fact-checking labels effect depend on the source to which the label is attributed?