Both studies were done in California: one among residents of Santa Clara County, south of San Francisco, and the other among residents of Los Angeles County. In both cases, estimates of the number of people infected across the country were much higher than the number of confirmed cases.
In the Santa Clara County study, the researchers tested 3,330 volunteers for antibodies indicating exposure. About 1.5% were positive. After adjusting to account for differences between the sample and the county’s population as a whole, the researchers estimated that the prevalence of antibodies was between 2.5% and just over 4%.
This means that between 48,000 and 81,000 people were infected in Santa Clara County in early April, the researchers concluded.
In Los Angeles County, researchers conducted two-day antibody tests at six driving test sites in early April and estimated that between 2.8% and 5.6% of the county’s adult population wore antibodies. If correct, it would mean that 220,000 to 442,000 residents were exposed.
By comparison, only 8,000 cases had been confirmed in the county at that time, testing was done.
Antibody studies in other countries have produced similar figures, noted Dr. John Ioannidis, a professor of medicine at Stanford University and author of the article on Santa Clara County.
If the numbers prove to be correct, he added, the virus could be much less deadly than expected, with a death rate more like that of a bad strain of influenza than a pandemic of deep lethality.
Neither report has been peer reviewed or published in a scientific journal, and both studies have been criticized. The two relied on volunteers, which may have skewed the results, and investigators say they are now probing their data to see the extent of this bias.