  The COVID-19 pandemic has caused major disturbance to human life. An
important reason behind the widespread social anxiety is the huge uncertainty
about the pandemic. A fundamental uncertainty is how many or what percentage of
people have been infected. There are published and frequently updated data on
various statistics of the pandemic, at local, country or global level. However,
due to various reasons, many cases were not included in those reported numbers.
We propose a structured approach for the estimation of the number of unreported
cases, where we distinguish cases that arrive late in the reported numbers and
those who had mild or no symptoms and thus were not captured by any medical
system at all. We use post-report data for the estimation of the former and
population matching to the latter. We estimate that the reported number of
infected cases in the US should be corrected by multiplying a factor of 220.54%
as of Apr 20, 2020, while the infection ratio out of the US population is
estimated to be 0.53%, implying a case mortality rate at 2.85% which is close
to the 3.4% suggested by the WHO in Mar 2020. Towards the end of the summer of
2020, the overall infection ratio of the US rises to 2.49% while the case
mortality decreases to 2.09%, and the ratio of asymptomatic cases out of all
infected cases reduces from the pre-summer 35-40% to around 20-25%.
