Researchers with USC in April tested the blood of 863 LA County residents. The results were analyzed and published this week in the Journal of the American Medical Association.
Those antibody tests examine whether people had the coronavirus in the past and had since developed antibodies that helped fight off the virus. Experts on coronavirus say many people can be infected and unknowingly carry the virus even when asymptomatic.
The collaborative study from USC and the county's department of public health found that more than 4% of those tested had antibodies to the coronavirus.
If those results were extrapolated to LA County's population of about 8 million adults, that means an estimated 360,000 adults in Los Angeles County had at some point gotten the infection, many of them without realizing it, the researchers say.
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If true, that would also mean the county's death rate per person from coronavirus is much lower than currently calculated.
And that result is from mid-April, when the county had only about 8,000 cases of COVID-19. As of this week, LA County is reporting more than 39,000 cases.
Neeraj Sood, a professor of health policy at USC, says even though the spread of COVID-19 through LA County is multiples higher than experts thought, the results still indicate we remain far from reaching herd immunity - where enough people have the antibodies that the virus begins running out of new hosts to infect.
"This is not something that is going to go away in the next month or the next two months," Sood said. "This is something that's going to be with us for several months or until the time we have a vaccine, which is probably 18 to 24 months away."
The study also found that about 60% of those who tested positive for antibodies were asymptomatic, showing few if any signs of an infection.