- the NYC antibody study one you mention, done at shopping centers, indeed likely over-represents people going out. 20%-21% of that population has antibodies.
- the SARS-CoV-2 testing study with pregnant women [1], tested just before delivery. Among those, 13% tested positive. It is reasonable to expect that this study over-represents subjects that barely go out: pregnant women to go out as little as possible to protect themselves and their future baby.
Because the sampling over-representation is opposite in the two studies, the truth is likely in between. in terms of antibodies, it is likely that the pregnant women that tested positive weeks ago have now developed antibodies or will do so in the next 1-2 weeks. Among pregnant women, the 13% also ignores women that had already developed antibodies before delivery; another phenomenon that may push the truth above 13%.