Facebook Pages, the "Disneyland" Measles Outbreak, and Promotion of Vaccine Refusal as a Civil Right, 2009-2019.

To understand changes in how Facebook pages frame vaccine opposition. We categorized 204 Facebook pages expressing vaccine opposition, extracting public posts through November 20, 2019. We analyzed posts from October 2009 through October 2019 to examine if pages' content was coalescing. Activity in pages promoting vaccine choice as a civil liberty increased in January 2015, April 2016, and January 2019 ([76] = 11.33 [ < .001]; [46] = 7.88 [ < .001]; and [41] = 17.27 [ < .001], respectively). The 2019 increase was strongest in pages mentioning US states ([41] = 19.06;  < .001). Discussion about vaccine safety decreased ([119] = -0.61;  < .001) while discussion about civil liberties increased ([119] = 0.33;  < .001]). Page categories increasingly resembled one another (civil liberties: [119] = -0.50 [ < .001]; alternative medicine: [84] = -0.77 [ < .001]; conspiracy theories: [119] = -0.46 [ < .001]; morality: [106] = -0.65 [ < .001]; safety and efficacy: [119] = -0.46 [ < .001]). The "Disneyland" measles outbreak drew vaccine opposition into the political mainstream, followed by promotional campaigns conducted in pages framing vaccine refusal as a civil right. Political mobilization in state-focused pages followed in 2019. Policymakers should expect increasing attempts to alter state legislation associated with vaccine exemptions, potentially accompanied by fiercer lobbying from specific celebrities.