Substantial regional variation
They found that 8.2% to 9.4% of the sample had COVID-19 antibodies, indicating exposure to the virus. After comparing the estimates with case counts from Johns Hopkins University, the researchers calculated that about 9.3% of the US population had been infected, with regional variation ranging from less than 5% in the West to more than 25% in the Northeast. Less than 10% of those with antibodies had been tested for COVID-19 while ill.
Residents of counties that had implemented lockdowns resulting in at least a 5% reduction in visits to workplaces in March were 60% less likely than others to test positive for coronavirus antibodies in July.
Patients from early US coronavirus hot spots had a significantly higher likelihood of having evidence of a previous infection (33.6% in New York, 17.6% in Louisiana, and 17.5% in Illinois). In contrast, residents of lesser affected neighboring states were less likely to have antibodies (6.4% in Pennsylvania and 1.9% each in Arkansas and Missouri).