Unfortunately, we don't know what titer level indicates a good response to vaccine or confers immunity in horses. There is currently no evidence that over-vaccinating in horses with any vaccine other than the Strangles vaccine causes health problems, so vaccinating as per your vet's/the AAEP's vaccination recommendations should be continued.
With dogs, the majority of the diseases we vaccinate against are more likely to occur in young dogs or don't carry a high mortality rate AND we have the studies to show that the vaccines are effective for longer than 1 year, thus the new recommendations to vaccinate less frequently in both dogs. We just don't have that situation with horses---EEE, Tetanus, WNV all are serious/deadly and can affect horses of any age. The other vaccines like Strangles, Influenza, EHV, PHF, etc are all given based upon the risk of the individual. Horse's vaccines schedules are already decided on an individual basis rather than just givin all horses all the vaccines yearly.
Licensed Veterinary Technician