  Although the online campaigns of anti-vaccine advocates, or anti-vaxxers,
severely threaten efforts for herd immunity, their reply behavior--the form of
directed messaging that can be sent beyond follow-follower
relationships--remains poorly understood. Here, we examined the characteristics
of anti-vaxxers' reply behavior on Twitter to attempt to comprehend their
characteristics of spreading their beliefs in terms of interaction frequency,
content, and targets. Among the results, anti-vaxxers more frequently conducted
reply behavior with other clusters, especially neutral accounts. Anti-vaxxers'
replies were significantly more toxic than those from neutral accounts and
pro-vaxxers, and their toxicity, in particular, was higher with regard to the
rollout of vaccines. Anti-vaxxers' replies were more persuasive than the others
in terms of the emotional aspect, rather than linguistical styles. The targets
of anti-vaxxers' replies tend to be accounts with larger numbers of followers
and posts, including accounts that relate to health care or represent
scientists, policy-makers, or media figures or outlets. We discussed how their
reply behaviors are effective in spreading their beliefs, as well as possible
countermeasures to restrain them. These findings should prove useful for
pro-vaxxers and platformers to promote trusted information while reducing the
effect of vaccine disinformation.
