https://www.nature.com/articles/d41586-020-01095-0

"An analysis of the blood of some 3,300 people living in Santa Clara county in early April found that one in every 66 people had been infected....... the researchers estimate that between 48,000 and 82,000 of the county’s roughly 2 million inhabitants were infected with the virus at that time — numbers that contrast sharply with the official case count of some 1,000 people reported in early April, not yet reveiwed"

This is a really inexpensive way to get an incredible amount of information,” says Jayanta Bhattacharya, a health economist at Stanford University in California and a co-author of the study.

The Santa Clara team estimated an IFR for the county of 0.1–0.2%, which would equate to about 100 deaths in 48,000-82,000 infections. As of 10 April, the county's official death count was 50 people. The study's IFR is lower than the IFR used in models by researchers at Imperial College London, which estimated an IFR for Great Britain on the basis of data from China to be 0.9%. In another study, the same group estimated an IFR for China of 0.66%, and a study of deaths on the Diamond Princess cruise ship estimated an IFR of 0.5%.