In the weeks since the 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.

Santa Clara County was an early epicenter 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 prevalence of the virus in the population.

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