There is no herd immunity to TB. USA eliminated most of it back in the mid 20th century with a single antibiotic before the resistant strands started spreading, which now requires a 4 drug cocktail for standard non-resistant strands now. We've just done a really good job of controlling it since then, it's not the fastest spreading disease out there. That said, the current volume of border crossings is probably going to cause an uptick here.

In countries with widespread BCG vaccination, substantial percentages of population are latently infected, and a smaller percentage of those will still become actively infected one day.

(Citing my sources

https://en.wikipedia.org/wiki/BCG_vaccine

https://thispodcastwillkillyou.com/2018/02/10/episode-9-tube...)