Could you have had COVID-19 as early as last year?

Since the COVID-19 pandemic began, the medical community has believed the virus was first identified in Dec. 2019. The first cases in the United States weren’t identified until Jan. 2020.

But a new study published in the journal Clinical Infectious Diseases suggests that the coronavirus, known officially as SARS-CoV-2, may have infected people in the U.S. as early as Dec. 2019.

Researchers analyzed blood donations collected by the American Red Cross from residents in nine states. They found evidence of coronavirus antibodies in 106 out of 7,389 blood donations. The Centers for Disease Control and Prevention analyzed blood that was collected between Dec. 13 and Jan. 17.

“The presence of antibodies in a person’s blood means they were exposed to a virus and their body’s immune system triggered a response,” explains Dr. Adam Treitman, Section Chief of Infectious Diseases and Medical Director of Infection Prevention, Advocate Christ Medical Center.

Initially, the first known case of COVID-19 in the U.S. was reported in Illinois on Jan. 24 from a woman who had returned from travel to China. The first case in Wisconsin wasn’t reported until Feb. 5. However, researchers found coronavirus antibodies in 39 blood samples from California, Oregon, and Washington as early as Dec. 13 to Dec. 16 of 2019. They also discovered antibodies in 67 samples from Connecticut, Iowa, Massachusetts, Michigan, Rhode Island, and Wisconsin in early January — before widespread outbreaks in those states.

“This study is very interesting though it appears to have some limitations,” Dr. Treitman said. “This particular Ortho total antibody test is a decent test, but some of the specimens do fall within the range of being a false positive result.”

Eric Beck, PhD, the Technical Director of Microbiology for Advocate Aurora Health’s ACL Laboratories agrees with Dr. Treitman that the study isn’t a “smoking gun.”

“There are also studies showing that antibody tests can become negative as fast as 90 days after infection, especially in mild illness,” Dr. Beck explained.

So, what does all this mean for people who had a cough, a fever and lost their sense of smell—common symptoms of the coronavirus— as early as Dec. of 2019? Although you might be convinced you had the disease, it’s still too early to know for sure.

“If patients believe they had COVID-19 late last year or early this year it’s not unreasonable for them to get tested for the antibodies. A positive result likely indicates a previous COVID-19 infection, but a negative result isn’t enough to say they didn’t have it,” he said.