Pro-Vaxxers Get Out: Anti-Vaccination Advocates Influence Undecided First-Time, Pregnant, and New Mothers on Facebook.

Social media has revolutionized health information-seeking behavior with crowd-based medical advice. Decreased vaccination uptake and subsequent disease outbreaks have generally occurred in localized clusters based on social norms; however, geographically unrestricted Facebook networks promote parental vaccination refusal congruent with digital identity formation. Interactions within the largest closed Facebook group for vaccination choice were analyzed through the lens of Social Influence Theory. Anti-vaccination advocates impacted first-time mothers' expressed vaccination intentions through both informational and normative influence processes. Six overarching themes were identified as strategies used by these individuals to persuade fence sitting parents to delay or decline vaccinations, including: natural solutions, maternal empowerment, distrust of conventional medicine establishment, fear appeals, 'Russian Roulette' risk benefit analysis, and misinformation and misunderstandings.