  COVID-19 poses disproportionate mental health consequences to the public
during different phases of the pandemic. We use a computational approach to
capture the specific aspects that trigger an online community's anxiety about
the pandemic and investigate how these aspects change over time. First, we
identified nine subjects of anxiety (SOAs) in a sample of Reddit posts ($N$=86)
from r/COVID19\_support using thematic analysis. Then, we quantified Reddit
users' anxiety by training algorithms on a manually annotated sample ($N$=793)
to automatically label the SOAs in a larger chronological sample ($N$=6,535).
The nine SOAs align with items in various recently developed pandemic anxiety
measurement scales. We observed that Reddit users' concerns about health risks
remained high in the first eight months of the pandemic. These concerns
diminished dramatically despite the surge of cases occurring later. In general,
users' language disclosing the SOAs became less intense as the pandemic
progressed. However, worries about mental health and the future increased
steadily throughout the period covered in this study. People also tended to use
more intense language to describe mental health concerns than health risks or
death concerns. Our results suggest that this online group's mental health
condition does not necessarily improve despite COVID-19 gradually weakening as
a health threat due to appropriate countermeasures. Our system lays the
groundwork for population health and epidemiology scholars to examine aspects
that provoke pandemic anxiety in a timely fashion.
