A Field Experiment Using Social Media Ads to Encourage COVID-19 Vaccination

Abstract Behavioral nudges in Facebook ads encouraged Covid-19 vaccine signup across six diverse countries. The ads reached nearly 14 million people and, consequently, many thousands took the concrete step of navigating to governments' vaccine signup sites. However, none of the treatment ads caused significantly more vaccine signup than placebo across all countries. Critically, reporting the descriptive norm that 87 percent of people worldwide had either been vaccinated or planned vaccination---social proof---did not meaningfully increase vaccine signup in any country and significantly backfired in South Africa and Taiwan. This result contradicts prominent prior findings. A charge to “protect lives in your family” significantly outperformed placebo in Brazil and Taiwan but saw mixed effects elsewhere. A message noting that vaccination significantly reduces hospitalization risk significantly increased signup in Brazil, Taiwan, and Turkey, had no significant effects in South Africa or Russia, and significantly backfired in the United States. Such heterogeneity was the the hallmark of the study: some messages saw significant treatment effects in some countries but failed in others. No nudge outperformed any other condition in Russia, a location of high vaccine skepticism. In all, widely touted behavioral nudges often fail to promote vaccine uptake and appear to be highly moderated by cultural context.