This is the first large scale antibody test having been done anywhere, to my knowledge. 3330 people were tested in Santa Clara California, and of those tested, 2.5% to 4.2% were found to have COVID-19 antibodies, meaning they were infected, asymptomatic and have since recovered and were never tested for COVID-19 itself.
“These prevalence estimates represent a range between 48,000 and 81,000 people infected in Santa Clara County by early April, 50-85-fold more than the number of confirmed cases,” the study’s abstract explained. “The population prevalence of SARS-CoV-2 antibodies in Santa Clara County implies that the infection is much more widespread than indicated by the number of confirmed cases,” according to the researchers. If the number of the recovered cases are similarly undercounted across the nation, the fatality rate would accordingly be overstated.
Of course this doesn't necessarily mean that the number of cases is 50-80x what's reported everywhere, but it's further proof of what many already suspected, which is that far more people have had the virus than what has been confirmed through testing, meaning the disease is far less fatal than the official numbers suggest.
Will be very interesting to see how this develops over time as more antibody tests are completed worldwide. The more people that have antibodies, the better it is for future virus transmission.