On Jun. 22, 2021, National Institutes of Health researchers reported that the prevalence of COVID-19 in the U.S. during spring and summer of 2020 far exceeded the known number of cases and that infection affected the country unevenly.

For every diagnosed COVID-19 case in this time frame, the researchers estimated that there were 4.8 undiagnosed cases, representing an additional 16.8 million cases by Jul. alone.