Sentiment, Contents, and Retweets: A Study of Two Vaccine-Related Twitter Datasets

"Understanding how vaccine-related information disseminates on Twitter is vital, especially because a minority of users are openly skeptical about vaccines and advocate against vaccination."

This study was carried out in the context of the challenge of communicating the benefits of vaccination to the public amid the presence of the antivaccination movement - in particular, as expressed through information shared on Twitter. This movement causes hesitance and criticism among parents regarding vaccines for myriad reasons, including lack of trust in government and the pharmaceutical industry, feared acute and long-term side effects, and concern over the chemical makeup of the vaccines themselves. In this article, the researchers report analyses of two distinct datasets that speak to this question: Do tweets with different sentiments toward vaccination and different contents attract different levels of Twitter users' engagement (retweets)?

In Study A, the researchers manually coded a 1% stratified random sample (N = 1,425) of 142,891 tweets (February 4 2010 to November 10 2016) with the hashtag #vaccine, a hashtag used by both provaccine and antivaccine advocates. They believed that tweets carrying stronger sentiments would attract more attention and retweets from those who wanted to share them.

Of the sample of 1,344 #vaccine tweets that were coded with vaccine-related sentiments, provaccine tweets accounted for 32.4% (436/1,344) of the sample, neutral tweets for 43.4% (583/1344), and antivaccine tweets for 24.2% (325/1,344). Among #vaccine tweets, provaccine tweets (adjusted prevalence ratio (PR) = 1.5836, 95% confidence interval (CI) = 1.2130-2.0713, p < 0.001) and antivaccine tweets (adjusted PR = 4.1280, 95% CI = 3.1183-5.4901, p < 0.001) had more retweets than neutral tweets. Specifically, antivaccine tweets and provaccine tweets were 4.1 and 1.6 times as likely, respectively, to be retweeted as were vaccine-related tweets with neutral sentiments. 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, or CDC (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%). The researchers say, "Given the use of Twitter across the opinion spectrum, it is not surprising that the top URL domain for both provaccine and antivaccine tweets was Twitter itself. In fact, it might reflect the growing trends that individuals rely on social media as their main source of news and information, compared with direct visits to Web sites of media or health organizations." Furthermore, these results add further evidence to the literature that people with antivaccine sentiment obtain and share information from alternative sources, probably because of their distrust of public health, medical, and pharmaceutical establishments. Therefore, simply releasing more scientific information online through websites and social media may not help.

One of the key findings from Study A is that "despite the provaccine health communication efforts made by public health agencies, as far as #vaccine tweets are concerned, on a tweet-by-tweet basis, antivaccine tweets may be receiving more attention (as reflected in the number of retweets) than provaccine tweets or neutral tweets. A potential explanation is that although the supporters of the antivaccine movements are a minority in the population, many of them are very committed to their cause and are active online....They retweeted tweets posted by like-minded individuals, forming an echo chamber..."

Study B involved analysis of a corpus of tweets with the hashtag #vaccineswork, which has been used by public health professionals when they promote vaccination. Because the distribution of retweet count is highly skewed with only very few tweets having high retweet count, it is likely that tweets with high retweet counts are read by many and may have influence over the knowledge, attitudes, or perceptions of many users. So, the researchers manually coded all 201 tweets with 100 or more retweets from 194,259 #vaccineswork tweets (January 1 2014 to April 30 2015). They provided a descriptive analysis of the distribution of topics among this sample of highly retweeted tweets and also combined several topics into a categorical variable to test if statistical association existed between retweet frequency and that categorical content variable.

Among highly retweeted #vaccineswork tweets, the most common theme was childhood vaccinations (40%; 81/201); 21% mentioned global vaccination improvement/efforts (42/201); and 29% mentioned vaccines can prevent outbreaks and deaths (58/201). To cite an example, second most retweeted tweet in this dataset was "*drops microphone* #antivax #vaccineswork #VaccinateYourKids http://t.co/1Nysbfkh7N" (retweet frequency = 5,032). Tweeted by @DocBastard, who described himself as a trauma surgeon in his user profile, this tweet ended with a link to an image of another physician's social media post about how he handled parents who declined to have their children vaccinated on schedule. This suggests that "having key opinion leaders who are active on social media to communicate our scientific message that 'vaccines work' is important, as it is through them that provaccine messages can reach users who normally would not follow social media accounts of health agencies."

In conclusion, the resarchers suggest that reaching out to key opinion leaders on Twitter to promote provaccine messages may help reach Twitter users who would be otherwise unreached by public health agencies.