  Public health measures were among the most polarizing topics debated online
during the COVID-19 pandemic. Much of the discussion surrounded specific
events, such as when and which particular interventions came into practise. In
this work, we develop and apply an approach to measure subnational and
event-driven variation of partisan polarization and explore how these dynamics
varied both across and within countries. We apply our measure to a dataset of
over 50 million tweets posted during late 2020, a salient period of polarizing
discourse in the early phase of the pandemic. In particular, we examine
regional variations in both the United States and Canada, focusing on three
specific health interventions: lockdowns, masks, and vaccines. We find that
more politically conservative regions had higher levels of partisan
polarization in both countries, especially in the US where a strong negative
correlation exists between regional vaccination rates and degree of
polarization in vaccine related discussions. We then analyze the timing,
context, and profile of spikes in polarization, linking them to specific events
discussed on social media across different regions in both countries. These
typically last only a few days in duration, suggesting that online discussions
reflect and could even drive changes in public opinion, which in the context of
pandemic response impacts public health outcomes across different regions and
over time.
