  In late-2020, many countries around the world faced another surge in number
of confirmed cases of COVID-19, including United Kingdom, Canada, Brazil,
United States, etc., which resulted in a large nationwide and even worldwide
wave. While there have been indications that precaution fatigue could be a key
factor, no scientific evidence has been provided so far. We used a stochastic
metapopulation model with a hierarchical structure and fitted the model to the
positive cases in the US from the start of outbreak to the end of 2020. We
incorporated non-pharmaceutical interventions (NPIs) into this model by
assuming that the precaution strength grows with positive cases and studied two
types of pandemic fatigue. We found that people in most states and in the whole
US respond to the outbreak in a sublinear manner (with exponent k=0.5), while
only three states (Massachusetts, New York and New Jersey) have linear reaction
(k=1). Case fatigue (decline in people's vigilance to positive cases) is
responsible for 58% of cases, while precaution fatigue (decay of maximal
fraction of vigilant group) accounts for 26% cases. If there were no pandemic
fatigue (no case fatigue and no precaution fatigue), total positive cases would
have reduced by 68% on average. Our study shows that pandemic fatigue is the
major cause of the worsening situation of COVID-19 in United States. Reduced
vigilance is responsible for most positive cases, and higher mortality rate
tends to push local people to react to the outbreak faster and maintain
vigilant for longer time.
