The preliminary study, conducted by researchers at Stanford University, estimates that between 2.5% and 4.2% of Santa Clara County residents had antibodies to the new coronavirus in their blood by early April. Antibodies are an indication that a person’s immune system has responded to a past infection.
In the weeks since the novel coronavirus outbreak has squelched daily life in America, researchers have struggled to assess the true spread of the virus. But initial results from a Northern California study on coronavirus antibodies suggest it has circulated much more widely than previously thought, according to a report released Friday.
The preliminary study, conducted by researchers at Stanford University, estimates that between 2.5% and 4.2% of Santa Clara County residents had antibodies to the new coronavirus in their blood by early April. Antibodies are an indication that a person’s immune system has responded to a past infection.
Though the county had reported roughly 1,000 cases in early April, the Stanford researchers estimate the actual number was between 48,000 and 81,000, or 50 to 85 times greater.
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Santa Clara County was an early hotbed of the
COVID-19 pandemic in California. By sampling the blood of 3,300 county residents and testing it for coronavirus antibodies, researchers hoped to estimate the prence of the virus in the population.
The antibody blood tests, also known as serology tests, are distinct from the diagnostic tests used to confirm an active infection.
Investigators from the Los Angeles County Department of Public Health and USC launched a similar study a week after Santa Clara began its effort. Officials say they will make their initials results public Monday.
Barbara Ferrer, the county’s top public health official, said that although the Los Angeles study will have somewhat different numbers, “they’re going to tell a similar story. And I think the story is … many more people have been infected with
COVID-19 than those who are being captured through our testing.”
The discrepancy, Ferrer said, owed to the slow start for coronavirus diagnostic testing and the fact that many people with few or no symptoms were unable to get tested.
There are important caveats to the Santa Clara findings. Unlike in Los Angeles County, where researchers recruited a random sample of participants to mirror the county’s makeup, the Northern California study solicited participants from social media.
As a result, white women ages 19 to 64 were overrepresented in the participant pool, while Latinos and Asian Americans were underrepresented relative to their share of the county’s population. Researchers tried to adjust for this imbalance in their analysis.
Also, the Stanford report was posted to a website for sharing early medical research findings and has not yet been vetted through the typical peer-review process.
Neeraj Sood, a professor at USC’s Price School of Public Policy who is an author of the Stanford report, said he was not surprised by the results.
“I didn’t know what the magnitude would be, but we definitely expected that there would be more cases than have been confirmed, just given the scarcity of testing,” said Sood, who is also the lead researcher from the Los Angeles County study.
One key takeaway of the Santa Clara County study is that a large number of people who are infected with the coronavirus never show any symptoms, said Karin Michels, chair of the epidemiology department at UCLA’s Fielding School of Public Health.
The fact that such people could unknowingly be contagious means that some level of physical distancing needs to remain in place to protect the elderly and people with underlying conditions who are much more endangered by the disease.
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