According to scientists who analyzed blood samples from American Red Cross donations, 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 thought.
The researchers found that they could not tell whether the apparent infections were among travelers who contracted the virus in other countries, or whether the infections resulted in wider community transmission.
Prior to this new report, the earliest documented infection in the country was on Jan. 19 from someone 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 coronavirus antibodies – protein markers for previous exposure towards the virus or maybe a very similar one.
At least one prominent virus researcher was concerned about how the results were interpreted online and in news reports. Trevor Bedford, an epidemiologist at the University of Washington who was heavily involved in genetic studies of how, when, and where the virus spread, said in a series of tweets that the study could identify people who had antibodies against other people had coronaviruses that cause colds, although he didn’t rule out some cases of travelers infected in other countries.
In the new report which was published online on Monday and was accepted for publication in the journal Clinical Infectious Diseases, Dr. Sridhar V. Basavaraju from the C.D.C. and others reported looking for antibodies that responded to the specific virus that caused the SARS-CoV-2 pandemic. They used samples of blood donated by the American Red Cross in nine states.
Susan L. Stramer, a virologist at the American Red Cross and a co-author on the paper, said the blood samples were originally taken to test exposure to mosquito-borne diseases such as West Nile virus. The C.D.C. analyzed the samples for signs of coronavirus exposure.
Dr. Stramer noted that the antibody tests are not intended for the virus itself and do not provide much useful information to the person whose blood is being tested. Antibodies can hang around in the blood after the virus leaves the body. But these blood markers can be useful for monitoring broad clinical pictures.
One problem with testing is that antibodies against certain coronaviruses, such as B. those that cause colds can also react to other viruses in the same family as SARS-CoV-2.
In the new tests on more than 7,000 samples, 106 showed coronavirus antibodies. The researchers narrowed this down to 84 who had antibodies that would attack or “neutralize” SARS-CoV-2 to some extent. One of these samples showed very effective neutralization, said Dr. Stramer. Another sample showed a reaction to part of the spike protein that is very specific for SARS-CoV-2. “For at least two samples, we think they are likely to be real infections.” However, these individuals could have been travelers infected outside of the United States.
In future studies, Dr. Stramer, researchers in earlier years will investigate whether blood samples would not show any antibodies against SARS-CoV-2 as expected.