  We estimate the growth in demand for ICU beds in Chicago during the emerging
COVID-19 epidemic, using state-of-the-art computer simulations calibrated for
the SARS-CoV-2 virus. The questions we address are these:
  (1) Will the ICU capacity in Chicago be exceeded, and if so by how much?
  (2) Can strong mitigation strategies, such as lockdown or shelter in place
order, prevent the overflow of capacity?
  (3) When should such strategies be implemented?
  Our answers are as follows:
  (1) The ICU capacity may be exceeded by a large amount, probably by a factor
of ten.
  (2) Strong mitigation can avert this emergency situation potentially, but
even that will not work if implemented too late.
  (3) If the strong mitigation precedes April 1st, then the growth of COVID-19
can be controlled and the ICU capacity could be adequate. The earlier the
strong mitigation is implemented, the greater the probability that it will be
successful. After around April 1 2020, any strong mitigation will not avert the
emergency situation. In Italy, the lockdown occurred too late and the number of
deaths is still doubling every 2.3 days. It is difficult to be sure about the
precise dates for this window of opportunity, due to the inherent uncertainties
in computer simulation. But there is high confidence in the main conclusion
that it exists and will soon be closed.
  Our conclusion is that, being fully cognizant of the societal trade-offs,
there is a rapidly closing window of opportunity to avert a worst-case scenario
in Chicago, but only with strong mitigation/lockdown implemented in the next
week at the latest. If this window is missed, the epidemic will get worse and
then strong mitigation/lockdown will be required after all, but it will be too
late.
