Expressions of pro- and anti-vaccine sentiment on YouTube.

Billions of hours of YouTube content are viewed every day. Much of this content is aimed at entertainment, some of it is educational, and a considerable quantity is meant to influence or reinforce public opinion on a variety of matters, including health. Most of the content on YouTube is not created by professionals, public institutions or the traditional media, and instead is authored by private individual content creators. Given the potential impact of this medium for communicating health information, it is important for researchers and public health practitioners to understand the nature of health information as it is shared on YouTube. The primary objective of this research is to describe expressions of vaccine hesitancy content on YouTube, and specifically, compare the expression of pro- and anti-immunization sentiments. We do this by not only analyzing a systematic sample of influenza and measles immunization videos in terms of viewer analytics, but also by choice of language. We find that pro- and anti-immunization videos are common, but that videos with anti-immunization sentiment tend to be more 'liked'. We also find that a small number of words can be effectively used to identify anti-immunization content, an observation that could be useful for identifying trends in anti-immunization sentiment on social media. Our results suggest that public health experts may need to increase the profile of their content on YouTube, and that there may be some useful strategies for improving the public's disposition towards pro-immunization messaging.