EXPRESS: Marketplaces of misinformation: a study of how vaccine misinformation is legitimized on social media

Combating harmful misinformation about pharmaceuticals spread through social media is a growing challenge. The complexity of health information, the role of expert intermediaries in disseminating information, and the information dynamics of social media create an environment where harmful misinformation spreads rapidly. However, little is known about the origin of this misinformation. This paper explores the processes through which health misinformation from online marketplaces is legitimized and spread. Specifically, across one content analysis and two experimental studies, we investigate the role of highly legitimized 'influencer' content in spreading vaccine misinformation. By analyzing a dataset of social media posts and the websites where this content originates, we identify the legitimation processes that spread and normalize discussions around vaccine hesitancy (Study 1). Study 2 shows that expert cues increase the perceived legitimacy of misinformation, particularly for individuals who generally have positive attitudes toward vaccines. Study 3 demonstrates the role of expert legitimacy in driving consumers’ sharing behavior on social media. This paper addresses a gap in our understanding of how pharmaceutical misinformation originates and becomes legitimized. Given the importance of the effective communication of vaccine information, the authors present key challenges for policy-makers