COVID-19 was spreading in the US by December 2019, mounting evidence suggests

The novel coronavirus may have already arrived in the U.S. by late December 2019, adding to the evidence that the virus was already lurking here at least a month before the country's first reported case, according to a new study.

The study researchers analyzed more than 24,000 blood samples collected in the U.S. from Jan. 2 to March 18, 2020, as part of the National Institutes of Health's All of Us program, a project that aims to gather health data from diverse populations.

The researchers identified seven participants from five states (Illinois, Massachusetts, Wisconsin, Pennsylvania and Mississippi) who tested positive for antibodies against the novel coronavirus, SARS-CoV-2, prior to the first reported cases of COVID-19 in those states. The earliest detection came from a sample collected Jan. 7 in Illinois. Because it takes about two weeks for a person to develop these antibodies, the new findings suggest the virus may have been circulating in Illinois as early as Dec. 24, 2019, according to the study, published Tuesday (June 15) in the journal Clinical Infectious Diseases.

"This study allows us to uncover more information about the beginning of the U.S. epidemic" of COVID-19, Dr. Josh Denny, CEO of All of Us and co-author of the study, said in a statement.

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The first confirmed case of COVID-19 in the U.S. was reported on Jan. 20, 2020, in a resident of Washington state who had recently traveled to China. But researchers have suspected that the virus arrived in the U.S. earlier than this, and a previous study from the Centers for Disease Control and Prevention also found evidence that the virus was in the U.S. in December 2019, Live Science previously reported.

Still, the new study has limitations. Although positive antibody tests suggest a prior infection with SARS-CoV-2, they cannot prove that a person had the virus. For example, even very accurate antibody tests still produce a small number of false positive results.

But the researchers did take steps to rule out false positives. They used two different antibody tests, and the samples had to test positive on both to be counted as positive results. The first test identified 147 samples that were positive for SARS-CoV-2 antibodies, but of these, only nine also tested positive on the second test. (Two of these samples were collected after the first cases of COVID-19 were identified in the participants' respective states.)   

There's also a chance that the samples had antibodies against other, similar coronaviruses that "cross-react" in this test to give positive results, but this is uncommon. In 1,000 blood samples collected in the U.S. from January to March 2019, which was used as a control group for comparison, none tested positive for antibodies against SARS-CoV-2.

"It is unlikely all nine of these [positive] individuals are false positive," the authors wrote.

It's important to note that the authors did not have information on whether the people who tested positive had traveled recently, which would suggest whether they became infected within their communities or elsewhere. The researchers plan to follow up with the people who tested positive to ask about their travel history, according to The New York Times. They are also planning further research to pinpoint exactly when the novel coronavirus first appeared in the U.S.

"The exact month at which it probably came into the U.S. is still unknown," study lead author Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told the Times.

Originally published on Live Science.