Behavioral nudges in social media ads show limited ability to encourage COVID-19 vaccination across countries.

Behavioral nudges in Facebook ads reached nearly 15 million people across six diverse countries and, consequently, many thousands took the step of navigating to governments' vaccine signup sites. However, none of the treatment ads caused significantly more vaccine signup intent than placebo uniformly across all countries. Critically, reporting the descriptive norm that 87% of people worldwide had either been vaccinated or planned vaccination-social proof-did not meaningfully increase vaccine signup intent in any country and significantly backfired in Taiwan. This result contradicts prominent prior findings. A charge to "protect lives in your family" significantly outperformed placebo in Taiwan and Turkey but saw null effects elsewhere. A message noting that vaccination significantly reduces hospitalization risk decreased signup intent in Brazil and had no significant effects in any other country. Such heterogeneity was the hallmark of the study: some messages saw significant treatment effects in some countries but failed in others. No nudge outperformed the placebo in Russia, a location of high vaccine skepticism. In all, widely touted behavioral nudges often failed to promote vaccine signup intent and appear to be moderated by cultural context.