I have no idea what the real numbers are, but I suppose that because of isolation and social distancing, at least 1/2 the population has had no exposure yet to CV-19.
To relax restrictions in the absence of a vaccine or a really effective treatment and without having demonstrated that the round 1 victims even have lasting immunity, is to kick the door open for round 2.
Since every city now has CV-19 carriers present, it could be a nationwide situation resembling Seattle before halting travel from China, NYC before St Patrick's Day or Nawlins before Mardi Gras. Are there enough new ventilators, masks, drugs etc. in place for that?