The virus may have arrived in the United States in December, but only spread later

Scientists analyzing blood samples taken from U.S. Red Cross donations said the coronavirus had already infected a few people in the United States on December 13, more than a month earlier.

The researchers noted that they could not say whether the apparent infections were in travelers who caught the virus in other countries or whether the infections were more widespread in the community.

Prior to this new report, the earliest documented infection in the country was on January 19, who traveled to China. Although other genetic studies suggest the virus may have occurred earlier than that date, the new study found that nine states sent to the Centers for Disease Control and Prevention had coronavirus antibodies in their blood donations — markers of previous exposure to the virus, or perhaps a very similar one.

At least one prominent virus researcher was wary of how the results were interpreted online and in the news. Trevor Bedford, an epidemiologist at the University of Washington who has been deeply involved in genetic studies of how, when and where the virus spread, said in a series of tweets he said the study could identify people who have antibodies to other human coronaviruses that have common colds. although it did not rule out that he may have picked up some cases of infected travelers in other countries.

In a new report, published online Monday and accepted for publication in the journal Clinical Infectious Diseases, Dr. Sridhar V. Basavaraju, CDC staff, reported looking for antibodies that respond to the specific virus that caused the pandemic, the SARS-CoV-2. Samples from blood donations collected by the American Red Cross in nine states were used.

Susan L. Stramer, a virologist at the American Red Cross and one of the authors of the article, said the blood samples were originally taken to test for diseases transmitted by mosquitoes, such as the West Nile virus. The CDC analyzed samples for evidence of coronavirus exposure.

Dr. Stramer noted that antibody tests do not apply to the virus and do not provide much useful information to the person whose blood is being tested. Antibodies can hang in the blood well after the virus has left the body. But these blood markers could be useful, he said, to monitor broad patterns of the disease.

One issue in testing is that certain coronaviruses, such as cold-causing antibodies, may also respond to other viruses in the same family, such as SARS-CoV-2.

In new studies of more than 7,000 samples, 106 showed coronavirus antibodies. The researchers narrowed this to 84, which had antibodies that attack or “neutralize” SARS-CoV-2 to some degree. One of these samples showed very effective neutralization, Dr. Stramer said. And another sample showed a reaction on a part of the spike protein that is very specific for SARS-CoV-2. “So for at least two samples, we think they’re probably real infections.” However, people who became infected outside the United States could have been travelers.

In future studies, Dr. Stramer said researchers in previous years were looking to see if the expected blood samples showed antibodies to SARS-CoV-2.