The study looked at more than 7,000 routine blood donation samples taken by the American Red Cross from people in nine states between December 13, 2019 and January 17, 2020.
A person's immune system develops antibodies when exposed to a pathogen like a virus to fight it off. Sixty-seven samples collected in Connecticut, Iowa, Massachusetts, Michigan, Rhode Island and Wisconsin in early January contained COVID-19 antibodies.
"The findings of this report suggest that SARS-CoV-2 infections may have been present in the United States in December 2019, earlier than previously recognized", the study said.
"Widespread community transmission was not likely until late February", the authors note.
That indicated that the infections were isolated by early this year.
Kamboj and colleagues used cell cultures to detect viable virus in serially collected respiratory samples from 18 recipient cancer patients who received hematopoietic stem-cell transplants or auto T-cell therapy, and two with lymphoma.
Delhi Police allows entry to farmers, gives permission for agitation in Burari
The agitating farmers also used a tractor to remove a truck placed as a barricade to stop them from entering Delhi. Farmers' bodies said they will hold a dharna wherever they are stopped from moving towards the national capital.
But the researchers wonder if the detection of antibodies in these patient samples really does indicate a past coronavirus infection, and not of another pathogen in the coronavirus family, such as the common cold.
Limitations of the research include the possibility of false-positive antibody tests.
They are - Level 4: Very high level of COVID-19; Level 3: High level of COVID-19; Level 2: Moderate level of COVID-19 and Level 1: Low level of COVID-19. In order to get a true positive, a different test would need to be a run.
Back in May, doctors in Paris also learned the coronavirus had been silently creeping around Europe a month before the official first-known cases were diagnosed in the region.
© Daniel Acker/Reuters A nurse dons personal protective equipment (PPE) as she prepares to enter the room of a COVID-19 patient being treated at UW Health University Hospital in Madison, Wis., on November 18, 2020.
Katie Camero is a McClatchy National Real-Time reporter based in Miami focusing on science.