It only takes once.
Will never give my own shots- New horse to me had the vet in probably only for shots, gave all my old/original guys. Left the newbie for last as I didn't know him or his behaviors. Gave the same as everyone else, 5 seconds allergic reaction on the ground in seizures. THANK GOD I had the vet there as he went to his truck and pulled out the right drug in the right amount and could get an IV in him in a matter of 30 seconds. That happened 23 yrs ago, still have that horse and still pay the farm call so- just in case...
And I too think that horses are over innoculated but, their living conditions,feeds,training programs,transportation,stresses all go against everything natural to them. If I lived in a place that could support 1 horse to 10 acres, they would be outside and mother nature would take care of them better than I can. But I don't so by owning them I also own the responsiblity to take care of them to the best ability and knowledge that I have.