Comment by jjjensen90

Because it's possible that there are many, many infected who don't seroconvert or ever manifest symptoms, and therefore would not have been tested with a PCR test, and would have a negative antibody test. We can see this in effect in studies of household attack rates and seroconversion. See this recent preprint and interesting thread (/r/covid19 is a strict science sub, not to be confused with /r/coronavirus) https://www.reddit.com/r/COVID19/comments/hdxwf5/intrafamili...