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The novel coronavirus may have been circulating in the United States undetected weeks before the first case was reported in 2020, a new government study suggests.

Study authors came to the conclusion after the U.S. Centers for Disease Control and Prevention found evidence of coronavirus antibodies in blood collected in December 2019, according to the report published Monday in the Clinical Infections Journal.

Researchers analyzed blood donations collected by the American Red Cross from residents in nine states between Dec. 13 and Jan. 17 and found evidence of antibodies in 106 out of 7,389 samples.

Antibodies also were found in 67 blood donations in January from states that didn’t report a widespread outbreak at the time, such as Connecticut, Iowa, Massachusetts, Michigan, Rhode Island and Wisconsin.

The first COVID-19 patient was diagnosed on Jan. 20 in Washington state, according to the CDC website. However, researchers found antibodies in blood sampled as early as Dec. 13.

“It’s possible that we had low level of prevalence earlier … and it’s not really surprising,” said Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security who was unaffiliated with the study.

Experts say the coronavirus could have been in the U.S. before the first case was recorded in January, but it’s also possible serological testing may have picked up a different strain or a different coronavirus altogether.

Antibodies are the body’s way of remembering how it responded to an infection so it can attack again if exposed to the same pathogen. The presence of antibodies in a person’s blood unique to the coronavirus suggests they may have been infected and recovered.

COVID antibodies: What is antibody testing and how does it work?

However, antibodies for SARS-CoV-2 look very similar to other coronavirus antibodies that cause illnesses, such as the common cold. Tests could mistakenly identify antibodies as being for the coronavirus that causes COVID-19, producing a false positive.

Critics could argue that is the case in the CDCstudy, Gronvall said, because levels for one of the antibodies, Immunoglobin M (IgM), are low for samples detected in December. The other antibody detected, Immunoglobin G (IgG), lasts longer in the body and was detected in higher levels but is less specific to the virus.

“Even a small percentage of inaccuracy can make a difference when you’re talking about these small numbers,” she said.

Study authors said they accounted for this limitation by using a very specific way of detecting the two virus-specific antibodies called microneutralization, but false positives are still possible. A true positive can only be confirmed with diagnostic testing.

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“We’re looking at the echoes of somebody’s infection,” Gronvall said. “I think we have pretty high expectations for the immediacy of our data and … the origin of diseases tends to be more murky and require months and years of investigation.”

While further research is needed, experts say the study highlights the importance of using serological testing for surveillance purposes to paint an accurate picture of how and where the virus is spreading.

“It has been a good way to see where we are in the pandemic … knowing how much of the population has already had it, trends over time, being able to provide care and direct resources,” Gronvall said. “That’s all really important.”

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

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