We took weanlings to Nationals for the first time this year. Our vet suggested an immune booster as opposed to giving vaccines. All three weanlings were given Zylexis. It was expensive, but well worth it and I will do it again this year, I will probably recommend it to all my clients as well. It is fine for those clients that don't want the added expense, but for me it really worked well. I felt the cost outweighed the possibility of my babies picking something up and me spending months trying to get them over it.
I know one person that purchased a horse and had it delivered to Nationals, we brought it home, it had a cough and a slight snotty nose at Nationals. She has been on all kinds of antibiotics etc and just can't seem to shake it. She has been home with the owner since Sept and she has one of the top vets in the NW working with her. The filly was a weanling.
The horses did not show any ill effects from the shot either.