The purpose of the study was to examine the characteristics of Internet memes created and disseminated by proponents and opponents of vaccinations. A quantitative content analysis was performed on 234 pro- and antivaccine memes culled from the vaccination fan pages with the greatest number of followers on Facebook. Coding variables included whether the meme was pro- or antivaccine, percentage of factually incorrect claims, mention of the out-group, persuasive appeals (emotion, fear, and rationality), degree of sarcasm, and number of reactions and shares. The most prevalent themes concerned vaccine-preventable diseases, vaccine injury/safety/autism, and conspiracy theories. Independent t tests indicated that provaccination memes were more likely to use sarcasm whereas antivaccination memes were more likely to contain emotion and fear appeals and inaccurate claims. The percentage veracity of the claims in each meme was fact-checked using authoritative scientific sources. A path analysis applying structural equation modeling revealed that memes containing characteristics that were antivaccine (vs. provaccine), appealed to emotion, and appealed to rationality significantly contributed to greater likelihood of social media reactions and shares. Additional analysis determined that both pro- and antivaccination memes tended to contain more gist than verbatim information, and both groups did not significantly differ on this gist-to-verbatim variable. Findings offer insights to understand the persuasion tactics that provaccine and antivaccine groups apply in memes to persuade others via social media. Understanding these techniques will enable the development of health communication strategies to combat false and damaging vaccine information disseminated on the Internet.