  In the age of social media, where billions of internet users share
information and opinions, the negative impact of pandemics is not limited to
the physical world. It provokes a surge of incomplete, biased, and incorrect
information, also known as an infodemic. This global infodemic jeopardizes
measures to control the pandemic by creating panic, vaccine hesitancy, and
fragmented social response. Platforms like Facebook allow advertisers to adapt
their messaging to target different demographics and help alleviate or
exacerbate the infodemic problem depending on their content. In this paper, we
propose a minimally supervised multi-task learning framework for understanding
messaging on Facebook related to the COVID vaccine by identifying ad themes and
moral foundations. Furthermore, we perform a more nuanced thematic analysis of
messaging tactics of vaccine campaigns on social media so that policymakers can
make better decisions on pandemic control.
