Earlier on Tuesday (June 15), the National Institutes of Health published a paper in "Clinical Infectious Diseases" showing that earlier cases of the new coronavirus were detected in five U.S. states (Illinois, Massachusetts, Mississippi, Pennsylvania and Wisconsin). Evidence of viral infection. The NIH study involved serum samples from 24,000 people in 50 U.S. states between January 2 and March 18, 2020. To avoid false positives and ensure the accuracy of the test, the researchers found samples from nine participants to be positive using two different serological tests.
https://www.bbc.com/zhongwen/simp/science-57498207