  Vaccine hesitancy has been recognized as a major global health threat. Having
access to any type of information in social media has been suggested as a
potential powerful influence factor to hesitancy. Recent studies in other
fields than vaccination show that access to a wide amount of content through
the Internet without intermediaries resolved into major segregation of the
users in polarized groups. Users select the information adhering to theirs
system of beliefs and tend to ignore dissenting information. In this paper we
assess whether there is polarization in Social Media use in the field of
vaccination. We perform a thorough quantitative analysis on Facebook analyzing
2.6M users interacting with 298.018 posts over a time span of seven years and 5
months. We used community detection algorithms to automatically detect the
emergent communities from the users activity and to quantify the cohesiveness
over time of the communities. Our findings show that content consumption about
vaccines is dominated by the echo-chamber effect and that polarization
increased over years. Communities emerge from the users consumption habits,
i.e. the majority of users only consumes information in favor or against
vaccines, not both. The existence of echo-chambers may explain why social-media
campaigns providing accurate information may have limited reach, may be
effective only in sub-groups and might even foment further polarization of
opinions. The introduction of dissenting information into a sub-group is
disregarded and can have a backfire effect, further reinforcing the existing
opinions within the sub-group.
