The coronavirus may have come to the United States earlier than previously thought.
The Center for Disease Control and Prevention has said the first COVID-19 cases in the U.S. were reported in mid-January.
However, the novel coronavirus could have started infecting people in the U.S. a month earlier, according to a study published on Monday.
CDC researchers analyzed blood donations collected by the American Red Cross from Dec. 13, 2019 through Jan. 17, 2020. There were more than 7,000 samples collected from nine states.
They were tested for reactive antibodies for the virus that cause COVID-19. Of the 7,000 tested, more than 100 were found to have antibodies.