Enlarge / Anti-vaccine advocate Robert F. Kennedy Jr. during a public hearing on vaccine related bills in 2015. (credit: Getty | Portland Press Herald) Just two organizations were responsible for the majority of anti-vaccine advertisements on Facebook before the social media giant restricted such content in March of this year, according to a November 13 study in the journal Vaccine. Of 145 anti-vaccine Facebook advertisements that ran between May 31, 2017, and February 22, 2019, the World Mercury Project and a group called Stop Mandatory Vaccination together ran 54% of them. The World Mercury Project, which ran the most ads of any single source, is an organization closely aligned with the anti-vaccine group Children's Health Defense.