When did the coronavirus arrive in the US?

The first infection was confirmed on January 21, 2020 in a Washington state resident who had recently returned from Wuhan, China. Shortly afterwards, experts concluded that the virus had been in the country for weeks.

A study published Tuesday provides new evidence: Based on an analysis of blood tests, scientists identified seven people in five states who may have been infected long before the first confirmed cases in those states. The results suggest that the virus was already circulating in Illinois, for example, on December 24, 2019, although the first case in that state was confirmed a month later.

But the new study is flawed, some experts said: it did not adequately address the possibility that the antibodies were against coronaviruses, which cause colds, and the results could be a quirk of the tests used. In addition, the researchers did not have any travel information for any of the patients, which may have helped explain the test results.

“This is an interesting paper because it raises the idea that everyone thinks it is true that there were infections that went undiagnosed,” said Scott Hensley, an immunologist at the University of Pennsylvania.

But the small number of samples that tested positive made it difficult to be sure that these were real cases of infection and not just a methodological error. “It’s hard to tell what is a real signal and what is not,” he said.

However, when the results are correct, they reinforce the idea that bad testing in the US missed most of the cases in the first few weeks of the pandemic.

“You can’t see what’s going on without testing,” said Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health and lead author of the study. “There have been a lot of infections in some of those states that we didn’t suspect in those earlier months.”

It is no surprise that there may have been undocumented cases at the start of the pandemic, said Sarah Cobey, an evolutionary biologist at the University of Chicago. Experts “already knew this was the case when they looked at trends in excess mortality and hospital admissions,” she said.

The latest model from Dr. Cobey estimated that there were about 10,000 infections in Illinois as of March 1, 2020. “Given the dire state of the tests, there was no doubt we missed the earliest broadcast,” she added.

In the study published in the journal Clinical Infectious Diseases, Dr. Althoff and her colleagues took blood samples from more than 24,000 people. They found nine people who donated blood between January 2 and March 18 last year and who appeared to have antibodies to the coronavirus.

Updated

June 15, 2021, 7:47 a.m. ET

Seven of the samples were from blood donated in their states of Illinois, Wisconsin, Pennsylvania, Mississippi, and Massachusetts prior to the date of initial diagnosis. The results agree with those of another study that identified coronavirus antibodies in donated blood as early as mid-December 2019.

Participants were enrolled in a long-term project by the National Institutes of Health called All of Us, which aims to involve one million people in the United States to increase minority representation in research. Only about half of the study participants were white.

At the beginning of the pandemic, the virus would have infected very few people. A low prevalence increases the likelihood that an antibody test will incorrectly identify a sample as an antibody when it doesn’t, said Dr. Hensley – a false positive.

The researchers tried to minimize this possibility by using two antibody tests in a row. The first test identified 147 samples as possible antibodies to the coronavirus; the second reduced that number to nine.

The team also analyzed 1,000 blood samples from the 2018/19 cold and flu season and found none that tested positive for antibodies to the coronavirus.

“It is still very possible that some of these are false positives,” said Dr. Josh Denny, CEO of All of Us. But “the fact that they would all be false positives seems pretty unlikely, given what we’ve done.”

The researchers said they planned to contact participants to inquire about travel history and would continue to analyze additional samples to estimate when the coronavirus hit American shores.

“The exact month it likely came to the US is still unknown,” said Dr. Althoff. “Right now, it’s essentially a puzzle, and our study is only part of that puzzle.”