  During 2020, social media chatter has been largely dominated by the COVID-19
pandemic. In this paper, we study the extent of polarization of COVID-19
discourse on Twitter in the U.S. First, we propose Retweet-BERT, a scalable and
highly accurate model for estimating user polarity by leveraging language
features and network structures. Then, by analyzing the user polarity predicted
by Retweet-BERT, we provide new insights into the characterization of partisan
users. Right-leaning users, we find, are noticeably more vocal and active in
the production and consumption of COVID-19 information. Our analysis also shows
that most of the highly influential users are partisan, which may contribute to
further polarization. Crucially, we provide empirical evidence that political
echo chambers are prevalent, exacerbating the exposure to information in line
with pre-existing users' views. Our findings have broader implications in
developing effective public health campaigns and promoting the circulation of
factual information online.
