  During the course of the COVID-19 pandemic, a common strategy for public
health organizations around the world has been to launch interventions via
advertising campaigns on social media. Despite this ubiquity, little has been
known about their average effectiveness. We conduct a large-scale program
evaluation of campaigns from 174 public health organizations on Facebook and
Instagram that collectively reached 2.1 billion individuals and cost around
\$40 million. We report the results of 819 randomized experiments that measured
the impact of these campaigns across standardized, survey-based outcomes. We
find on average these campaigns are effective at influencing self-reported
beliefs, shifting opinions close to 1% at baseline with a cost per influenced
person of about \$3.41. There is further evidence that campaigns are especially
effective at influencing users' knowledge of how to get vaccines. Our results
represent, to the best of our knowledge, the largest set of online public
health interventions analyzed to date.
