Chernobyl is a great example of this with deadly consequences. COVID is far worse sadly.
I don't think we could have contained it within China if they had a best response it was already out - well maybe 99.99% chance a not that educated guess - but we could probably have given ourselves a good amount more time. And hard shut down borders quickly if we knew the true vast numbers & death rates in early Wuhan. Then put out the 'embers' locally.
Gives at least some countries a chance of keeping it under control within their borders. Though I don't have faith our (US) CDC would have been up to the task...