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Researchers can test blood samples taken for other reasons to see if patients have previously had COVID-19. Don Bartletti/Los Angeles Times via Getty Images

COVID-19 official counts can miss mild cases – here’s how serosurveys that analyze blood for signs of past infection can help

Your blood can hold a record of past illnesses. That information can reveal how many people have had a certain infection – like 58% of Americans having had COVID-19 by the end of February 2022.

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