Let’s move up patient zero to November 10. There are scenarios in which he could be spreading it around immediately after being inoculated - through his physical contact with the virus and him physically touching things right after.
He would also be transmitting the virus through droplets after the virus has replicated itself in him. Patient zero may have been asymptomatic.
November 10 this all happens in my simulation here.
In a population area like Wuhan. Let’s say he successfully inoculated 50 people a day. Let’s say he inoculated them through droplets (so after the virus replicated itself in him). Let’s put this at November 15th he starts inoculating people with great frequency.
Those 50 people start successfully inoculating people on November 20th. That’s now 2500 inoculated people in a population rich area by November 25th.
Thanksgiving is a major travel holiday across the world. Lots of traffic in and out of the US. Many young people in China celebrate it too.
So by Thanksgiving travel, those 2500 inoculated people could give it to 50 people each in a population rich area.
This virus had to have made landfall in the US in early to mid December. And a chance even earlier had one of the first batch of inoculated people ran into an international traveler.