US likely to have had SARS cases in December 2020
Simon Collins, HIV i-Base
A paper published in CID looking at retrospective testing of more than 24,000 samples collected during the first months of 2020 from 50 US states before testing for SARS-CoV-2 was easily available.
Of theses, 9 were seropositive, 7 of whom predated the first confirmed case in the states of Illinois, Massachusetts, Wisconsin, Pennsylvania, and Mississippi.
The authors report that their findings indicate SARS-CoV-2 infections weeks prior to the first recognised cases in 5 US states.
Althoff KN et al. Antibodies to SARS-CoV-2 in All of Us Research Program Participants, January 2-March 18, 2020, Clinical Infectious Diseases, 2021. (15 June 2021).