This piece is fascinating and in the context of future localised outbreaks in the UK its slightly scary.
www.nytimes.com/interactive/2020/us/coronavirus-spread.html
How the virus won
HOW THE VIRUS GOT IN
The China travel ban was a partial success: Only a handful of infected travelers from China are estimated to have made it into the country undetected before restrictions were imposed on Feb. 2.But it wasn’t enough.
A vast wave of infected travelers — roughly 1,000, one model suggests — came from other countries in Asia, Europe and the rest of the world in February, each a dangerous spark that could set off a wider outbreak.
Many of those infections died out. But by mid-February, a few caught fire and became outbreaks, spreading invisibly.
And
Researchers with the Seattle Flu Study ignored C.D.C. testing restrictions and uncovered a single case with no travel history in late February. This was the first sign that the outbreak had spun badly out of control.Over the two weeks that followed, people made about 4.3 million trips from the Seattle area.
Thousands were contagious. Genetic samples linked to the Seattle outbreak appeared in at least 14 states, said Trevor Bedford, a professor at Fred Hutchinson Cancer Research Center and a leader of the flu study.
Seattle was just the beginning. In New York City, where officials had found only a single case by March 1, roughly 10,000 infections had spread undetected.
New Yorkers and visitors continued to travel out of the city. More than 5,000 contagious travelers left in the first two weeks of March, estimates suggest.
And
By the time President Trump blocked travel from Europe on March 13, the restrictions were essentially pointless.The outbreak had already been spreading widely in most states for weeks.
Definitely worth a read.