Both studies were performed 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, the estimates of the number of people infected countrywide were far higher than the number of confirmed cases.

In the Santa Clara County study, researchers tested 3,330 volunteers for antibodies indicating exposure. Roughly 1.5 percent were positive. After adjustments intended to account for differences between the sample and the population of the county as a whole, the researchers estimated that the prevalence of antibodies fell between 2.5 percent and a bit more than 4 percent.

That means that between 48,000 and 81,000 people were infected in Santa Clara County by early April, the researchers concluded.

In Los Angeles County, researchers conducted antibody tests for two days at six drive-through test sites in early April and estimated that between 2.8 percent to 5.6 percent of the county’s adult population carried antibodies. If accurate, that would mean that 220,000 to 442,000 residents have been exposed.

By comparison, only 8,000 cases had been confirmed in the county at that time the testing was done.

Antibody studies in other countries have produced similar numbers, noted Dr. John Ioannidis, a professor of medicine at Stanford University and an author of the paper on Santa Clara County.

If the numbers prove accurate, he added, the virus may be much less deadly than originally expected, with a fatality rate more closely resembling that of a bad flu strain than a pandemic of profound lethality.

Neither report has been peer-reviewed or published in a scientific journal, and both pieces of research have met with criticism. Both relied on volunteers, which may have skewed the results, and the investigators say they are now probing their data to see how significant this bias may have been.

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