Even if we are talking 20%. That is 70 million Americans. We aren’t anywhere near that number
Actually we might be close to that number...I have read estimates from several sources that the actual number of people in the US that have had (or currently have) COVID-19 is tracking at about 10x-12x the currently tested cases count (6,211,796 tested positive cases for the US). So maybe about 68 million that have already had COVID-19 in the USA. (20% of the current USA Pop. is about 66.27 million people).
https://theweek.com/speedreads/922103/cdc-estimates-may-have-actually-more-than-20-million-covid19-cases (June numbers)
https://www.worldometers.info/coronavirus/#countries
Number infected is much greater than the number of positive tests. Many never get tested. The way you tell is you test antibodies in a random population. NYC and the northeast is already there.