(CNN) Far more people may have been infected with Covid-19 than have been confirmed by health officials in Santa Clara County, California, according to a study released Friday in preprint.

The study used an antibody blood test to estimate how many people had been infected with Covid-19 in the past. Other tests, like those performed with nasal swabs or saliva, test for the virus' genetic material, which does not persist long after recovery, as antibodies do.

"We found that there are many, many unidentified cases of people having Covid infection that were never identified with it with a virus test," said Dr. Jay Bhattacharya, a professor of medicine at Stanford University and one of the paper's authors . "It's consistent with findings from around the world that this disease, this epidemic is further along than we thought."

The study estimated that 2.49% to 4.16% of people in Santa Clara Country had been infected with Covid-19 by April 1. This represents between 48,000 and 81,000 people, which is 50 to 85 times what county officials recorded by that date: 956 confirmed cases.

A preprint study is a draft version of a study released to the public that hasn't been peer-reviewed for publication in a journal.

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