If you look closely at the numbers of shadows who the MC fight we can deduce that "the mist" engulfed at maximum a city and the surroundings
In the other case we should had hordes of millions of former humans who hunted us to death
I guess the rest of mankind after realising the mortality of the misterious mist and the improbability of survivors, knowing that in that region there was a military laboratory involved in strange experiments, resolved to simply quarantine the area instead of a costly and useless research