starting off with a fresh paper on early transmission in US
https://www.medrxiv.org/content/10.1101/2020.07.06.20140285v1
>We use a global metapopulation transmission model to study the establishment of sustained and undetected community transmission of the COVID-19 epidemic in the United States. The model is calibrated on international case importations from mainland China and takes into account travel restrictions to and from international destinations. We estimate widespread community transmission of SARS-CoV-2 in February, 2020. Modeling results indicate international travel as the key driver of the introduction of SARS-CoV-2 in the West and East Coast metropolitan areas that could have been seeded as early as late-December, 2019. For most of the continental states the largest contribution of imported infections arrived through domestic travel flows.
tl;dr there was undetected spreda since late jan / early feb in NY and CA (just like how Trevor Bedford said on Feb 8 already: https://twitter.com/trvrb/status/1226241657860870144 ) and US response was incompetent









