Actually, @theaustrianguy was incorrect, and @chron was correct.

Every one of the tourists is an unknown regarding vaccination, and therefore needs to be counted as unvaccinated for the purpose of calculating herd immunity.

Every one of them could have dropped off a disease upon their visit, so they were all potential vectors, that decreased the immunity of the herd in direct proportion to their numbers.

However, they aren't members of the herd at all, they don't stay. Their number cannot be added to the herd.

Their impact compromises the immunity of the herd, but their transitory nature means they cannot be considered a part of the herd.

@chron was correct from the get go, and the good doctor wrong.