The new study, which hasn't yet been peer reviewed, tested 3,300 residents of Santa Clara County for antibodies that would indicate they've been infected with COVID-19.
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When the samples were taken, on April 3 and 4, the number of confirmed coronavirus cases in Santa Clara County was around 1,100. (That number has since surpassed 1,800.) The total population of the county is 1.9 million, which would mean a total infection rate of about 0.06%.
Based on the prevalence of antibodies found in the 3,300 people tested, the Stanford researchers believe the actual proportion of the population who has been infected with the new coronavirus is somewhere between 2.5% and 4.2%.
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That would mean the actual number of people infected in Santa Clara County could be anywhere from 48,000 to 80,640.
"Our findings suggest that there is somewhere between 50- and 80-fold more infections in our county than what's known by the number of cases than are reported by our department of public health," Dr. Eran Bendavid, the Stanford professor who headed the study, told ABC News' Diane Sawyer.
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"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," the study concludes.
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The study hasn't stood the test of peer review, a process that can take a few weeks or several months.
It's no surprise the number of actual cases would trump the official count, as testing has been limited not just in Santa Clara County, but globally. New York City recently recently revised its official death toll to reflect an additional 3,700 people who are presumed to have succumbed to the virus, but were never tested.
ABC7 News reached out to the Santa Clara County Public Health Department for comment on the study. We will update this story when we hear back.