  For more than a decade, doubt about vaccines has become an increasingly
important global issue. Polarization of opinions on this matter, especially
through social media, has been repeatedly observed, but details about the
balance of forces are left unclear. In this paper, we analyse the flow of
information on vaccines on the French-speaking realm of Twitter between 2016
and 2017. Two major asymmetries appear. Rather than opposing themselves on each
vaccine-related controversy, pro and anti-vaccine accounts focus on different
vaccines and vaccine-related topics. Pro-vaccine accounts focus on hopes for
new groundbreaking vaccines and on ongoing outbreaks of vaccine-preventable
illnesses. Vaccine critics concentrate their posts on a limited number of
controversial vaccines and adjuvants. Furthermore, vaccine-critical accounts
display greater craft and energy, using a wider variety of sources, and a more
coordinated set of hashtags. This double asymmetry can have serious
consequences. Despite the presence of a large number of pro-vaccine accounts,
some arguments raised by efficiently organized and very active vaccine-critical
activists are left unanswered.
