I don't even usually vaccinate for it, because mine don't have contact with other horses much, perhaps only on trail rides. It's very similar to kennel cough in dogs. I gave KC vaccine to my dog who was actively showing, but not as a matter of course otherwise.
Well, from what it sounded like, your horse could get it just from being where another horse had been who had it, for several months, so if you take any of them off your place at all, then they are all at risk.