Participants who reported antibodies against SARS-CoV-2 were likely exposed to the virus at least several weeks before their sample was taken as the antibodies do not appear until about two weeks of infection, the researchers said.
The samples came from Illinois, Massachusetts, Mississippi, Pennsylvania and Wisconsin, and were part of a study of more than 24,000 samples taken for a National Institutes of Health research program between January 2 and March 18, 2020.
Of the seven samples, three were from Illinois, where the first confirmed coronavirus case was reported on January 24, while the remaining four states had one case each. Samples from participants in Illinois were collected on January 7 and Massachusetts on January 8.
Coronavirus antibodies were detected in nine of the 24,000 samples, all of which belonged to people living in areas outside Seattle and New York City, where scientists believed the coronavirus first entered.