It usually doesn't say it has to be, it just uses multiple continents as an example of a large geographic area. Of course, there are multiple different definitions out there.
Regardless, it's silly to assume the virus would stay in the US.
even if you achieve herd immunity, there will always be breakthrough cases, as with all pandemics. vaccines without respirators, contact tracing, purifying the air, etc etc will lead to the same catastrophic results as covid.
most countries abandoned basic health measures for covid, and that should alarm everyone for the next pandemic