  Opposition and hesitancy to vaccination have been one of the major threats to
global health. Social media sites have been suspected as a breeding ground of
misleading narratives about vaccines, but little is known about how pervasive
anti-vaccine views are on the world's largest social media, Facebook. Here, we
study the prevalence of online groups on Facebook with anti- and pro-vaccine
views and the information ecosystem that enables the production and
dissemination of anti-vaccine narratives, using the largest collection of
Facebook data on this issue to date. The results reveal that 74% of posts each
month on average were generated by anti-vaccine groups. It is also shown that
anti-vaccine groups had increasingly more relied on relatively credible sources
while their posts using low credibility sources were less than 2% and recently
decreasing. Furthermore, anti-vaccine groups depended more on their exclusive
sources and utilized sources representing more conservative or far-right
political views than pro-vaccine groups did. The findings suggest that
expansive and targeted interventions are urgently needed to curb the
circulation of online narratives against vaccination.
