Covid-19 was spreading in the us by december 2019

The study researchers analyzed more than 24,000 blood samples collected in the U.S. from Jan. 2 to March 18, 2020, as part of the National Institutes of Health's All of Us program, a project that aims to gather health data from diverse populations.
The novel coronavirus may have already arrived in the U.S. by late December 2019, adding to the evidence that the virus was already lurking here at least a month before the country's first reported case.
Serologic Testing of US Blood Donations to Identify Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)–Reactive Antibodies: December 2019–January 2020.
Of the 7389 samples, 106 were reactive by pan-Ig. Of these 106 specimens, 90 were available for further testing. Eighty-four of 90 had neutralizing activity, 1 had S1 binding activity, and 1 had receptor-binding domain/ACE2 blocking activity >50%, suggesting the presence of anti–SARS-CoV-2–reactive antibodies. Donations with reactivity occurred in all 9 states.These findings suggest that SARS-CoV-2 may have been introduced into the United States prior to 19 January 2020.
The median time from SARS-CoV-2 infection to IgG antibody presence is 14 days.13–15 Assuming individuals who were seropositive according to the sequential definition were infected at least 2 weeks prior to biospecimen collection, our findings suggests the virus may have been present in Illinois as early as December 24, 2019.