NIH study suggests coronavirus may have been in U.S. as early as December 2019 The Washington PostMore evidence suggests COVID-19 was in US by Christmas 2019 Yahoo NewsScientists Report Earliest Known Coronavirus Cases in Five US States The New York TimesStudy: COVID-19 circulating in U.S. weeks before before first case KTNV Channel 13 Las VegasNIH antibody study suggests COVID-19 spread across U.S.