if there's multiple viruses, antibody testing would also indeed be unreliable - in the sense that you're only testing for one virus. and, you could look at that either way...

are these antibody tests being based on chinese data or european data? i know that a lot of the kits came from china.

if we're testing for the chinese virus in new york, and find that 30% of the people tested have antibodies, that is an intriguing result, but may ultimately have little to do with the outbreak that occurred, which was in the european virus. i've been scratching my head wondering why the numbers are so low. it could be that they're testing for the wrong virus, and that an antibody test for the european strain would produce even higher antibody prevalence.