  We report a large-scale randomized controlled trial designed to assess
whether the partisan cue of a pro-vaccine message from Donald Trump would
induce Americans to get COVID-19 vaccines. Our study involved presenting a
27-second advertisement to millions of U.S. YouTube users in October 2021.
Results indicate that the campaign increased the number of vaccines in the
average treated county by 103. Spread across 1,014 treated counties, the total
effect of the campaign was an estimated increase of 104,036 vaccines. The
campaign was cost-effective: with an overall budget of about \$100,000, the
cost to obtain an additional vaccine was about \$1 or less.
