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A new study found coronavirus may have been in the US in December. That doesn’t mean you’ll ever know if you had it then

A lab technician sorts blood samples inside a lab for a COVID-19 vaccine study at the Research Centers of America (RCA) in Hollywood, Florida, on August 13, 2020. Here in Utah, University of Utah Health researchers are fundraising to study the effects of COVID-19 long-term. (Photo by CHANDAN KHANNA/AFP via Getty Images)

(CNN) — A new study published Monday suggests the novel coronavirus was infecting people across the US as early as December — a month before the first person known to have been infected with coronavirus arrived in the US from China on January 15.

Researchers screened blood donations made in December and early January and found evidence of antibodies to the novel coronavirus in at least 84 samples from nine states — something that would suggest those people had been infected with the coronavirus.

“These findings suggest that SARS-CoV-2 may have been introduced into the United States prior to January 19, 2020,” the researchers at the US Centers for Disease Control and Prevention and the Red Cross wrote in the journal Clinical Infectious Diseases.

Many people wonder if they may have had coronavirus early in the pandemic, or even before people knew the virus was circulating.

It would be hard to know unless someone took a coronavirus test at the time — something that obviously did not exist. The symptoms are similar to those of many other flu-like infections: a cough, a fever, body aches, perhaps a sore throat.

And without a sample taken at the time of infection or soon after, it will be impossible to know whether an individual was infected back then, said Dr. Ian Lipkin, an infectious diseases specialist at Columbia University.

Antibody tests can indicate a past infection, but not when that infection occurred.

“You cannot distinguish between somebody being infected in December or being infected in March or April,” Lipkin told CNN.

A person’s immune response to a viral infection such as Covid-19 changes over time. Several studies have shown that antibody responses ramp up right after infection and then change over time. It’s not yet known how long the human body maintains an immune response to coronavirus infection.

“There are tests that in the first few weeks of infection indicate very recent infection, but after that you can’t really tell,” Lipkin said. The only exception would be if someone had taken a blood test earlier that showed they were negative for the virus.

But the study shows that blood donations may be an important source of information about when the virus began circulating, the CDC and Red Cross researchers said. “These findings also highlight the value of blood donations as a source for conducting SARS-CoV-2 surveillance,” they wrote in Tuesday’s report. SARS-CoV-2 is the scientific name for the coronavirus that causes Covid-19.

Blood donations stored since last year could be tested for antibodies, although the type of test could be important. Several other coronaviruses cause common colds and tests could mistakenly detect infections with those viruses, the CDC and Red Cross researchers noted.

“We clearly want to know how long this virus has been circulating,” Lipkin added.

“People are trying to figure out when this first appeared outside China.”

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