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The test used to detect the virus are so sensitive that they can detect fragments of virus still present in the blood.
Those tested may have had the disease months ago.
They keep testing people that they havent tested before and detect people that no longer have the disease but have fragments in their blood.
The article also points out that since COVID is related to SARS and the common cold that false positives could be due to someone having the other viruses.
Having had the other viruses could also confer some immunity to COVID.