They can't be contained, but time can be bought to develop a vaccine, and that time is measured in lives.

With fully closed borders, Australia and NZ managed to have no increase in overall mortality. South Korea didn't even close borders, just imposed entry quarantines with strong test and trace, and was able to obtain a much lower net mortality than North America and Europe also.

just to note; fully closing borders was not possible for most countries, so while it may help, it was never realistic

Sure, but every barrier to movement reduces transmissions, makes contact tracing easier, and buys more time for vaccine distribution.