alt Hacker NewsFirst case in December 2019. February 2, 2020 fogging machines on city streets in China: https://www.youtube.com/watch?v=vLdwCdyiDCw
We were definitely reading about the lockdowns in Wuhan in January 2020.
January 19, 2020, the first case in Washington state was detected in a man who had recently traveled from Wuhan.
Surveillance blood samples from Dec 13, 2019 to January 17, 2020 from several nine US states were tested for COVID-19 antibodies: "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"