Abstract
COVID-19 vaccines are widely available in wealthy countries, yet many remain unvaccinated. We report on two studies (United States and France) with millions of Facebook users that tested two strategies central to vaccination outreach: health professionals addressing common concerns and motivating “ambassadors” to encourage vaccination in their social networks. We can reject very small effects of any intervention on new first doses (0.16 pp, United States; 0.021 pp, France), with similar results for second doses and boosters (United States). During the Omicron wave, messaging aimed at the unvaccinated or those tasked with encouraging others did not change vaccination decisions.
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