A government study released on Tuesday found that several people in five states had COVID-19 antibodies before those states confirmed their first cases of the virus last year, suggesting that the virus was spreading undetected in the U.S. as early as late 2019.
Researchers analyzed blood samples from more than 24,000 people that were collected as part of the National Institutes of Health (NIH) All of Us study and found seven people who tested positive for antibodies as early as Jan. 7, all before states had reported their first COVID-19 infections.
The study, published as an accepted manuscript in the journal Clinical Infectious Diseases, determined that people in Illinois, Massachusetts, Wisconsin, Pennsylvania and Mississippi had COVID-19 antibodies before the states confirmed any cases.
The blood samples, taken between Jan. 2 and March 18, 2020, underwent two COVID-19 antibody tests that identified whether certain antibodies that appear at least two weeks after infection were present. Researchers considered the samples to have antibodies if both tests returned positive results.
Nine people total tested positive for antibodies, including the seven whose positive tests came before the first confirmed case in their state. Seven of the nine were from minority populations.
The presence of these antibodies in some participants suggests that they were infected with COVID-19 several weeks before their blood sample was taken.
“This study allows us to uncover more information about the beginning of the U.S. epidemic and highlights the real-world value of longitudinal research in understanding dynamics of emerging diseases like COVID-19,” Josh Denny, CEO of All of Us and an author of the study, said in a release.
The earliest positive antibody tests were among people in Illinois and Massachusetts on Jan. 7-8, 2020, respectively, indicating they were exposed in late December.
The U.S. confirmed its first COVID-19 infection on Jan. 21, 2020, with Illinois identifying its first case on Jan. 24, 2020, and Massachusetts announcing a presumptive COVID-19 case on March 2, 2020.
Blood from one person from Wisconsin and one from Pennsylvania also tested positive for antibodies in early February, even though Wisconsin confirmed its first case about two days later and Pennsylvania announced its first presumptive case in early March.
Another person from Mississippi had their blood taken in early March, about five days before the state confirmed its first case.
The research supports a previous Centers for Disease Control and Prevention (CDC) study that indicated the COVID-19 virus may have been spreading in the U.S. as early as December 2019.
The scientists noted that some states’ sample size was low and that it’s unknown if the participants who tested positive for antibodies were exposed while traveling or in their communities.