The virus may have arrived in the United States in December, but did not spread until later

© Misha Friedman for The New York Times
Blood samples from Covid-19 patients at a Brooklyn lab earlier this year.

The coronavirus may have infected a small number of people in the United States as early as December 13, more than a month earlier than researchers had thought, according to scientists who analyzed blood samples taken from donations from the American Red Cross.

The researchers noted that they could not say whether the apparent infections were from travelers who had caught the virus in other countries, or whether the infections had led to wider community transmission.

Before this new report, the first documented infection in the country was on January 19 in a person who had traveled to China. Although other genetic studies have suggested the possible presence of the virus before that date, the new study found that blood donations from nine states sent to the Centers for Disease Control and Prevention contained anti-coronavirus antibodies – protein markers of ‘past 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 news reports. Trevor Bedford, a University of Washington epidemiologist who has been deeply involved in genetic studies of how, when and where the virus spread, said in a series of tweets that he thought the study could identify people who had antibodies against other humans. coronavirus, which cause colds, although he did not rule out that he could have detected some cases of infected travelers in other countries.

In the new report, which was published online Monday and was accepted for publication in the journal Clinical Infectious Diseases, Dr. Sridhar V. The Pandemic, SARS-CoV-2. They used samples taken from donated blood that the American Red Cross had collected in nine states.

Susan L. Stramer, a virologist at the American Red Cross and one of the authors of the article, said the blood samples were originally collected to test for exposure to mosquito-borne diseases like West Nile virus. The CDC analyzed the samples for evidence of exposure to the coronavirus.

Dr Stramer noted that antibody tests are not for the virus itself and do not offer a lot of useful information to the person whose blood is being tested. Antibodies can stay in the blood long after the virus has left the body. But these blood markers can be useful, she said, in monitoring broad disease trends.

One problem with testing is that antibodies to certain coronaviruses, such as those that cause the common cold, may also respond to other viruses in the same family, such as SARS-CoV-2.

In new tests on more than 7,000 samples, 106 showed anti-coronavirus antibodies. The researchers reduced that number to 84 which contained antibodies that would attack or “neutralize” SARS-CoV-2 to some extent. One of those samples showed very efficient neutralization, Dr Stramer said. And another sample showed a reaction to a part of the spike protein which is very specific to SARS-CoV-2. “So for at least two samples, we think they probably represent real infections.” However, these people could have been infected travelers outside of the United States.

In future studies, Dr Stramer said, researchers will look at previous years to see if the blood samples, as expected, show antibodies to SARS-CoV-2.