Introduction: Social media platforms are important channels through which health education about the utility and safety of vaccination is conducted.

Objective: To investigate if tweets with different sentiments toward vaccination and different contents attract different levels of Twitter users' engagement (retweets).

Methods: A stratified random sample (N = 1425) of 142,891 #vaccine tweets (February 4, 2010, to November 10, 2016) was manually coded. All 201 tweets with 100 or more retweets from 194,259 #vaccineswork tweets (January 1, 2014, to April 30, 2015) were manually coded. Regression models were applied to identify factors associated with retweet frequency.

Results: Among #vaccine tweets, provaccine tweets (adjusted prevalence ratio = 1.5836, 95% confidence interval = 1.2130-2.0713, p < 0.001) and antivaccine tweets (adjusted prevalence ratio = 4.1280, 95% confidence interval = 3.1183-5.4901, p < 0.001) had more retweets than neutral tweets. No significant differences occurred in retweet frequency for content categories among antivaccine tweets. Among 411 links in provaccine tweets, Twitter (53; 12.9%), content curator Trap.it (14; 3.4%), and the Centers for Disease Control and Prevention (8; 1.9%) ranked as the top 3 domains. Among 325 links in antivaccine tweets, social media links were common: Twitter (44; 14.9%), YouTube (25; 8.4%), and Facebook (10; 3.4%). Among highly retweeted #vaccineswork tweets, the most common theme was childhood vaccinations (40%; 81/201); 21% mentioned global vaccination improvement/efforts (42/201); 29% mentioned vaccines can prevent outbreaks and deaths (58/201).

Conclusion: Engaging social media key opinion leaders to facilitate health education about vaccination in their tweets may allow reaching a wider audience online.