New study suggests COVID-19 arrived in the U.S. last December without anyone knowing

It’s widely recognized that the first clinically diagnosed case of COVID-19 in the U.S. was in Washington state in late January. But a new study suggests the virus could have been circulating here as far back as early to mid-December 2019 without anyone realizing it.

The study, which was conducted by researchers at the Centers for Disease Control and Prevention and published in the journal Clinical Infectious Diseases, analyzed blood donations from the American Red Cross that were collected from nine states. The researchers discovered evidence of coronavirus antibodies in 106 of the 7,389 blood donations, collected between Dec. 13 and Jan. 17.

Antibodies are proteins made by the immune system to fight infections like viruses, per the Food and Drug Administration. They may also help prevent future illness from those particular infections. The presence of certain antibodies in the blood suggests that a person was exposed to a particular virus or bacteria — in this case COVID-19, Dr. Amesh A. Adalja, senior scholar at the Johns Hopkins Center for Health Security, tells Yahoo Life.

The researchers found antibodies to SARS-CoV-2, the virus that causes COVID-19, in 39 samples from California, Oregon and Washington state dating back to Dec. 13, 2019. Antibodies were also found in 67 blood samples taken in Connecticut, Iowa, Massachusetts, Michigan, Rhode Island and Wisconsin in early January.

“These findings suggest that SARS-CoV-2 may have been introduced into the United States prior to January 19, 2020,” the researchers concluded.

The researchers also said their findings suggest that “isolated SARS-CoV-2 infections may have occurred in the western portion of the United States earlier than previously recognized or that a small portion of the population may have pre-existing antibodies” to the virus.

There is a big “but” here, though.

The researchers noted in the paper that there have been previous cross-reactive antibody responses between SARS-CoV-2 and some other common human coronaviruses. “The antibodies we found could have been from SARS-CoV-2 infections or from common coronavirus infections,” study co-author Natalie J. Thornburg, a researcher at the CDC, tells Yahoo Life.

“Some antibody tests, especially those with lower specificity for SARS-CoV-2 in particular, might yield false positive results because of cross-reactivity with other coronaviruses that cause the common cold,” Dr. Prathit Kulkarni, assistant professor of medicine in infectious diseases at Baylor College of Medicine, tells Yahoo Life.

However, the authors did try to control for this by weeding out certain types of coronaviruses. “Some but not all of the positive samples are very strongly positive to SARS-CoV-2 by multiple tests, and therefore more likely to be from SARS-CoV-2 infections,” Thornburg says.

As a result, the study can’t definitively say whether there were cases of COVID-19 circulating in the U.S. before late January. This may be difficult to prove, Kulkarni says. “Without widespread testing in January and February of 2020, it is difficult to know precisely the extent to which SARS-CoV-2 might have been spreading around the United States at that time,” he says.

Still, many experts think the virus was present before it was actually detected.

“There clearly were cases before the first ones were recognized,” Adalja says. “This study gives a lot of clues, but there is the one big caveat that there could be antibodies to other seasonal coronaviruses.”

Adalja says the study results are “suggestive” that the virus had been in the U.S. for weeks before it was detected, but “we still need more data.”

Dr. Richard Watkins, an infectious disease physician in Akron, Ohio, and a professor of internal medicine at the Northeast Ohio Medical University, agrees. “It seems reasonable to conclude” that the virus was present in the U.S. before the end of January, he tells Yahoo Life. “It is a new disease, and at the time there were no lab tests to diagnose it available in the U.S.,” he points out.

As for why the virus didn’t spread rapidly in the U.S. sooner if it was, in fact, here, Adalja says the cases may simply have been “dead-end infections.” He adds, “Not every case is going to spread.”

It’s also possible that the virus was widely circulating in the country but was being written off as the cold or flu. “It may not have gotten noticed, especially during cold and flu season,” Adalja says.

Another factor to consider, per Adalja: Cases of a particular virus often don’t get detected until they start causing hospitalizations. “There need to be enough cases in vulnerable populations that land people in hospitals to get us to notice,” he says. It’s possible that many early cases of COVID-19 didn’t cause severe disease and, as a result, didn’t alarm medical professionals, Adalja says.

That makes a big difference, Dr. Lewis S. Nelson, professor and chair in the Department of Emergency Medicine at Rutgers New Jersey Medical School, tells Yahoo Life. “If you’re not looking for it, you’re not going to find it,” he says. “But when we went out looking for it, we found it.”

Thornburg says it’s difficult to know how widespread the virus was in the U.S. in December and early January. “The data does not suggest wide circulation, but instead potential sporadic rare cases,” she says.

While there still is some uncertainty on exactly when the virus started circulating in the U.S., experts generally agree it was sooner than most people realize. “Many people in the field think that there were likely COVID-19 cases in the U.S. prior to January,” Adalja says.

Determining the exact date when the virus arrived in the U.S. doesn’t have a huge impact on things now, Kulkarni says. “From the practical standpoint, it does not change much of what we should be doing now, continuing usual public health measures and rolling out our national vaccination plan,” he says.

Dr. Peter Winklestein, executive director of the Institute for Healthcare Informatics, Jacobs School of Medicine and Biomedical Sciences at the University at Buffalo, tells Yahoo Life it also doesn’t increase the odds of developing herd immunity for the virus. “There’s nothing that supports that,” he says. “The latest findings are an interesting factoid, but we’re nowhere near herd immunity. If you think you had the virus last December, you sure as heck shouldn’t consider yourself Superman.”

For the latest coronavirus news and updates, follow along at According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

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