Covid-19: Do many people have pre-existing immunity?
The ship epidemics give an indication of whether this is the case. The USS Teddy Roosevelt had an epidemic and everyone onboard was tested. 20% tested positive for the virus. 60% showed antibodies.Arctic cruise ship after 21 days: "Of the 217 passengers and crew who remained on the ship for the entirety of the voyage, 128 tested positive for the coronavirus, the researchers said. They added that of those who tested positive, 24 exhibited symptoms and 104, or 81%, did not."[2]There are many other ship epidemics, but those are ones that had 100% testing. 
That puts a ceiling on pre-existing immunity at somewhere around 40%. No floor from that data.[1] https://www.cnn.com/2020/06/09/health/covid-19-investigation...[2] https://www.cnbc.com/2020/05/27/over-80percent-of-coronaviru...