COVID-19 antibody testing

Data suggest that about 3% of Americans, or nearly 10 million people, have been infected with coronavirus. Unfortunately, this data comes from late April and early May, and the virus has spread even further since then. The official COVID-19 case tally, therefore, is a dramatic undercount.
To understand how severe and lethal COVID-19 really is, we need to know how many have been infected, which, in this equation, is the "denominator." An early study from Stanford of Santa Clara County says we may be underestimating how many cases there already are, which inaccurately gauges COVID-19’s infectivity and eventual mortality.