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Although the earliest cases of coronavirus were first identified in the United States this past January, the findings of a government study published Monday suggest that people may have first been infected in December.

The conclusion reached by scientists at the Centers for Disease Control and Prevention bolsters prior evidence that the virus may have begun spreading across the globe earlier than first believed — including one earlier model from researchers at Northeastern University that projected more than 100 people in Boston had likely been exposed by mid-February.

The first known case of coronavirus was detected in Wuhan, China, in December 2019. In the United States, the first infection was reported in mid-January. Two other patients in the US had illness onset dates of Jan. 14, 2020, according to the study, which was published in the journal of Clinical Infectious Diseases.

Boston, meanwhile, was one of the first cities nationwide to report that someone had been infected with the virus: a student from the University of Massachusetts Boston who had traveled from Wuhan.

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But the new study suggests that the virus may have been present earlier than that.

“Some reports have suggested the introduction of SARS-CoV-2 into the [United States] may have occurred earlier than initially recognized, though widespread community transmission was not likely until late February,” scientists wrote.

To determine whether antibodies from SARS-CoV-2 — the virus that causes COVID-19 — were present prior to the first identified case in the United States, researchers tested archived samples from 7,389 routine blood donations collected by the American Red Cross.

“Serologic testing has been previously used to estimate introduction of viral infections into populations, including for HIV,” scientists wrote.

The samples analyzed were gathered by the organization during the period from Dec. 13, 2019, through Jan. 17, 2020, from donors who lived in nine different states: California, Connecticut, Iowa, Massachusetts, Michigan, Oregon, Rhode Island, Washington, and Wisconsin.

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Of the more than 7,000 samples tested by scientists at the CDC, 106 — or 1.4 percent — had evidence of infection, according to the study.

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

But the study also found “detection of reactive antibodies” in the specimens tested, indicating that the virus may have surfaced even earlier.

For instance, there was potentially cross reactivity with human coronavirus infection other than SARS-CoV-2, according to the study.

Based on data collected, however, scientists wrote, “at least some of the reactive blood donor sera could be due” to previous infection from the virus.

Scientists addressed other limitations of the study as well, including the fact that none of the sera can be considered “true positives.” Furthermore, the donations included in the report “may not be representative of all blood donors or donations in these states and the findings may not be generalizable to all blood donors during the donation dates reported here,” scientists wrote.

As a result, population-based estimates or inferences on the magnitude of infections on a “national or state level” cannot be made, according to the study.

Additional studies involving the analyses of human specimens are needed to “further corroborate the present findings,” which detected the presence of antibodies specific to SARS-CoV-2 as early as mid-December 2019, scientists wrote.

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The CDC is continuing to partner with both federal and non-governmental partners “to conduct ongoing surveillance using blood donations and clinical laboratory samples for SARS-CoV-2 infection in multiple sites,” across the country, according to the study.


Shannon Larson can be reached at shannon.larson@globe.com. Follow her on Twitter @shannonlarson98.