First, remember that the spring vaccination trip is often the only trip horses make to a vet each year and it is the basis for the required "doctor-client-patient" relationship that allows your vet to give advice at other times during the year without seeing your horse and to dispense other meds that you might need like Bute or Banamine to keep on hand. Horses also need their teeth checked at least once yearly to look for issues and determine if floating is needed so you are killing several birds with one stone.
As for what to give, there are multiple different 5-way and 4-way vaccines on the market and they don't all protect against the same diseases. You need to know specifically what is in each vaccine and make sure you are protecting your horses against the most common and deadly diseases---EEE, WEE, WNV, Tetanus and Rabies. And in many states an owner cannot vaccinate against rabies----a veterinarian has to do it. Then you also need to consider the risk factors for EVH, Influenza, Potomac Horse Fever, Strangles, Botulism, etc and make sure you are vaccinating against everything that is recommended for your horse's situation and boostering often enough. EHV and Influenza vaccines aren't just once yearly vaccines. Discuss this with your vet and get his recommendations for your particular situation.
As for how much to give, all horses are given the same mount irregardless of size. My boss did adjust the dose strangles vaccines for mini's, but there are no studies to show that it is necessary and under-dosing is going to affect the protection the vaccine gives.
Licensed Veterinary Technician
Last edited by Ryle; 05-17-2009 at 12:15 PM.