As individuals turn to social media sites for health information, it is important to understand the factors that influence their perceptions of this information. This study employed an experiment to investigate the impact of social media discussion on perceptions of vaccines. Analysis, using structural equation modeling, revealed that source credibility and the civility of a discussion indirectly influence policy support requiring vaccinations, intention to vaccinate future children, and willingness to engage in discussion about vaccines. This impact is mediated by message elaboration and attitudes of the value of vaccinations. As individuals think about a pro-vaccine message and understand the importance of receiving vaccinations, they are more likely to engage in pro-social vaccination choices.