This is known for certain in Seattle. The first case was in mid-January[1], then six weeks passed before the big outbreak started and genetic sequencing showed that the virus was directly descended from that original case six weeks earlier[2], indicating that there had been ongoing transmission throughout the community for the past six weeks.
[1] https://www.seattletimes.com/seattle-news/health/snohomish-c...
[2] https://www.washingtonpost.com/health/coronavirus-may-have-s...
ps be sure to read that first link, published in january. It's absolutely maddening how cavelier everyone was being, even while China was literally shutting down a city the size of new york and sealing off the entire province...
I see this accusation thrown around a lot, but it doesn't align with timeline and import statistics.
APT32 (Vietnam) hacked China as early as Jan 6th, but didn't shutdown borders until late Jan, well after WHO confirmed H2H and after China the broadcasted seriousness of covid19 with Wuhan followed by country wide lockdowns. Apart from shutting borders with Hubei/China in late Jan - early Feb, the countries that responded well shared one common quality: they immediately rolled out effective screening, testing, tracing and isolation. Which is in line with WHO recommendations on Jan 23 and Jan 30.
The effectiveness of border closing is debatable, since imported cases statistics from these regions (SK, HK, Taiwan, Singapore) indicate very few cases were exported out of China (less than 30), most of which were out of Hubei, whereas hundreds of imported cases came from Americas and Europe. Australia, New Zealand and Canada is similar. These are all places with massive traffic flows with China that also repatriated hundreds to thousands from Wuhan with few positives cases that spread was or could have been managed without huge impacts on civic life. Vietnam is different in that it doesn't have as much resources as other countries so it must necessarily ere on the side of caution.
But broadly, the countries that didn't handled the pandemic well didn't listen to WHO and China and waited too long before acting.
I say this because reading this article, they mention that Vietnam quarantined an entire district in Hanoi back in February when the entire country only had 10 cases at that point in time. They also banned travel from and to mainland China in early February. Those measures done early may have saved them from tens of thousands of infections. Vietnam has essentially relied on aggressive contact tracing coupled with strict lockdowns/quarantining.
Here in Canada, we brushed off suggestions of forced quarantines of incoming travelers or banning flights to/from Mainland China. We solely relied on travelers telling authorities if they experienced symptoms, which bit us in the tail when March came along and hundreds of new cases were popped up by the day.
We didn't reverse our stance on restrictive measures till the virus had gotten out of a control. Even now as the growth rate has flattened, we're still dealing with the fallout in long term care facilities as hundreds of elderly people have passed away due to caregivers/visitors unknowingly infecting them. That coupled with the growth of the virus in the homeless community has made it very challenging to sustainably contain the virus.
I think what frustrates me is that we knew we had weaknesses in our system, yet we decided to go with the non-proactive solution anyway. Even recently we didn't bother with random testing asymptomatic caregivers until after hundreds of people died. It's a lot harder to undo the damage than it is to be overly pre-cautious and prevent the worst case scenario from happening in the first place.