  For efficient political decision-making in an emergency situation, a thorough
recognition and understanding of the polarized topics is crucial. The cost of
unmitigated polarization would be extremely high for the society; therefore, it
is desirable to identify the polarizing issues before they become serious. With
this in mind, we conducted a retrospective analysis of the polarized subtopics
of COVID-19 to obtain insights for future policymaking. To this end, we first
propose a framework to comprehensively search for controversial subtopics. We
then retrospectively analyze subtopics on COVID-19 using the proposed
framework, with data obtained via Twitter in Japan. The results show that the
proposed framework can effectively detect controversial subtopics that reflect
current reality. Controversial subtopics tend to be about the government,
medical matters, economy, and education; moreover, the controversy score had a
low correlation with the traditional indicators--scale and sentiment of the
subtopics--which suggests that the controversy score is a potentially important
indicator to be obtained. We also discussed the difference between subtopics
that became highly controversial and ones that did not despite their large
scale.
