It was a novel coronavirus, it takes time to study the virus after it's discovery. Initially they thought it might have been spread through food, or by animals. They were unsure and had to confirm the method transmission to be sure. US intelligence however, had already suspected it was spread from person to person in late November. Suspecting and confirming are entirely different things here. Due to the earlier strains of the virus not being as contagious as the strain in Europe means it would also likely take longer in the lab to determine all it's possible methods of transmission. This is also why they initially didn't think you could give it to your pets, but found out that too was inaccurate once it mutated to the much more contagious strain. China was not testing the same strain that we were dealing with in the rest of the world, and thus would have different lab results.
Researchers have found that the strains spreading so quickly in Europe and the U.S. have a mutated S “spike” protein that makes it about 10 times more infectious than the strain that originally was identified in Asia.
www.biospace.com
It was Initially the strain of SARS-CoV-2 that was in Wuhan was not as contagious as the strain that was taking over Europe and the US. The US was primarily infected from Europe by a variant, not directly rom China and not by the same strain of the virus. The earlier strains that were in China are not the primary strains in the rest of the world.
The coronavirus variant, known as 20A.EU1, has been identified in 12 European countries as well as in Hong Kong and New Zealand, scientists said.
www.businessinsider.in
We actually have unknown viruses pop up all the time all over the world, but they usually fizzle out or are not serious so we do not pay much attention to them. The 2009 swine flue outbreak was circulating for months prior to being discovered. They actually think it could have been circulating as far back as September before being discovered in late March:
“The limited sampling so far gives rise to considerable uncertainty in the estimate,” cautions Rambaut. But if the rate at which genes mutate is about the same for this virus as for other H1N1 viruses, the number of mutations that have accumulated so far suggests it has been circulating since January – or even September 2008."
Mutation rates show the H1N1 flu virus may have jumped from pigs to humans as long ago as January – but also hint that it may be easy to snuff out
www.newscientist.com
A previously unknown swine flu virus is spreading in people in the US and Mexico, where it has killed at least 18
www.newscientist.com
www.cdc.gov
It took them a very long time to discover it circulating.