Thanks for citing that recent serological study which presents evidence that SARS-CoV-2 was circulating the U.S. as early as 13-16 December 2019 (here's a slightly better link [1]).
FWIW here is another reference with similar findings [2], and also a paper (cited in the OP) suggesting infections were massively under-reported early in the pandemic [3].
> By mid-July of 2020, 16.8 million people had an undiagnosed SARS-CoV-2 infection, almost five times the rate of diagnosed infections. [2]
> I wonder why this study has not been repeated for blood samples collected before December 2019?
Speculating but it's probably because such research is expensive and knowing the exact date of the first infection may not be particularly useful.
[1] Serologic Testing of US Blood Donations to Identify Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)–Reactive Antibodies: December 2019–January 2020
https://academic.oup.com/cid/article/72/12/e1004/6012472
FWIW here is another reference with similar findings [2], and also a paper (cited in the OP) suggesting infections were massively under-reported early in the pandemic [3].
> By mid-July of 2020, 16.8 million people had an undiagnosed SARS-CoV-2 infection, almost five times the rate of diagnosed infections. [2]
> I wonder why this study has not been repeated for blood samples collected before December 2019?
Speculating but it's probably because such research is expensive and knowing the exact date of the first infection may not be particularly useful.
[1] Serologic Testing of US Blood Donations to Identify Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)–Reactive Antibodies: December 2019–January 2020 https://academic.oup.com/cid/article/72/12/e1004/6012472
[2] Identification of the first COVID-19 infections in the US using a retrospective analysis https://assets.researchsquare.com/files/rs-707353/v1/baa84b8...
[3] Undiagnosed SARS-CoV-2 seropositivity during the first 6 months of the COVID-19 pandemic in the United States https://stm.sciencemag.org/content/13/601/eabh3826