There have been a couple antibody studies in CA. The one in Santa Clara County found 1.5% of 3300 people had antibodies, in California's supposed "ground zero". In LA they tested 900 people and found 4.1% with antibodies. It makes sense that NY would be much, much higher than anywhere in CA.
I would take all of these antibody studies with a grain of salt. I don't trust these tests that have been rushed out, and some of the more capable "big pharma" companies have held back on releasing AB tests due to high false positives when sensitivity is high enough to reduce false negatives to an acceptable level. When actual population with antibodies is in the low single digits, you need better than 99% confidence in your positive results without producing false positives.
Also there was other problems with both the Chinese test kits used and major conflict of interests on the part of the authors.
My working theory is that pretty much everyone has antibodies, if we look hard enough, given that the virons were racing through this country since January (if not much earlier). The question really is whether these people who test positive have ENOUGH antibodies to prevent an infection if again exposed, and to a higher concentration.
Easy way to find out. Get some FluBros with antibodies and send them to a Coronavirus ward for an hour or so, unmasked, and see if they get sick. If they do get sick, maybe they’ll learn something from the experience.