2% of blood samples collected in California in December 13-16th, 2019 analyzed in this study had antibodies reactive to SARS-CoV-2. It’s possible some are due to cross reactivity of antibodies from other corona viruses but not likely that all were.
https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1785/6012472

Antibody development (especially IgG) lags SARS-CoV-2 infection, so it seems likely that the virus was here as early as late November.

The tests found IgM antibodies as well, which indicates active or recently active infection among those donating blood in December.

So the interesting question is why do US deaths due to COVID-19 only begin in March? Before the blue-check-marks-with-pronouns go nutso on me, I’m not contesting the death counts. I’m just saying we need to understand what’s going on here.