The Covid-19 virus infected people in five U.S. states in December 2019 and early 2020 before those states reported their first cases, according to a large new government study, providing new insights into the first, unseen weeks of the nation’s deadly epidemic.
Scientists analyzing blood samples taken for a National Institutes of Health research program identified seven people in states from Mississippi to Wisconsin to Pennsylvania who were infected with the new virus days or weeks before the first cases were confirmed in their areas. At least a couple had mild symptoms.
Their findings were published online by the journal Clinical Infectious Diseases Tuesday.
Two samples, one taken from a person from Illinois and another from a person from Massachusetts, date to Jan. 7 and 8, 2020, respectively, the researchers said. Antibodies found in the samples appear about two weeks after a person has been infected, the researchers said.
The number of Covid-19 cases found in the frozen, stored blood samples is small, suggesting the early cases in the U.S. were sporadic.
All told, the researchers found evidence of infection in just nine out of 24,079 participants whose blood samples were taken between Jan. 2, 2020, and March 18, 2020, for the NIH research program.
Still, the findings provide evidence the new coronavirus was infecting Americans before the world learned that it was causing a deadly outbreak in Wuhan, China. Two of the cases also occurred weeks before public-health officials confirmed the virus had arrived in the U.S., and before they started testing people widely.
"It helps us understand a little bit more about the geographic spread of where the virus was in those very early days of the U.S. epidemic," said Keri Althoff, lead author and associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health in Baltimore.
The cases the researchers found, Dr. Althoff said, suggest the new virus was "being seeded perhaps in states that we didn’t necessarily have on our radar before this."