If I had access to a test kit, I would go by  titers.  For an IR/EMS horse other consideration is vaccine reactions. A horse that is  in their teens or older and has been vaccinated yearly more than likely has a very high titers. Many horses were vaccinated more often than that because encephalitis vaccines used to all contain tetanus. In the past, when USDA ran titers, we had members test their horses and find high titers. When titers aren't available, it's the better part of valor to vaccinate but if the horse is having reactions the risk vs benefit ratio may shift toward not vaccinating.
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Eleanor in PA

www.drkellon.com 
EC Owner 2001