
An estimated 16.8 million U.S. adults had undiagnosed COVID-19 by July 2020, representing 4.6% of U.S. adults and 4.8 undiagnosed infections for every diagnosed case, according to a National Institutes of Health study published by the journal Science Translational Medicine. NIHâs SARS-CoV-2 Seroprevalence Project used an enzyme-linked immunosorbent assay to identify SARS-CoV-2 antibodies in dried blood samples collected from a representative sample of U.S. adults.