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The US has vaccines, but not enough to instantly vaccinate 8 billion + people. 
At the end of the day a plan is plan and that's all it is. A lot can happen that leads to better or worse outcomes than nations have planned for.
Best case: Maybe a 1918 type situation.
Worst case: Societal collapse type event.
Most likely, something in between the two.
I wonder how prepared we are. Bird flu is substantially harder to contain than Coronavirus.