They must be vacinated before they are one year ( 12 months ) old by a certified vet. My Father started that program after we lost the whole herd in Nebraska in the late 70's to bangs. We have had no problems since.
(Knock on wood.)
Our neighbors just laughed at him about it down here when he first moved down, but now most of them are doing it also.
An Ounce of precaution is better than buying a Pound of (USDA) Cure.