When I was a young girl, no one I knew who had horses gave them any immunizations. I remember they had a farrier in now and again, but most didn't receive immunizations, and most didn't see a vet. Most of us are doing a much better job of caring for our horses today.
In my region, there are a number of immunizations that should be administered, at least, annually. If you have a horse who has never been immunized, is ill, or you live in an area where certain mosquito borne illnesses are more common, then your vet may direct you to give certain immunizations to your horses every six months, or occasionally every three. Some horses who travel or who are in the company of many other horses, such as in shows, may also have his immunizations increased as needed under the direction of an equine vet.
Even though I am a registered nurse and have given thousands of intramuscular, subcutaneous, intravenous, and intradermal injections to people over the years, animals are another matter. I have been giving my dogs DHLPP injections under vets instructions for years. I have been giving my alpacas their immunizations and worming injections since the 1990s. However, horses are entirely another matter. Give a horse injection improperly and it may not work as desired. Administer it in the wrong place and you will do damage or traumatize the horse. Approach the horse incorrectly and give the injection improperly and you may well be injured or killed by a spooked horse ! So, rather than learning to give an intramuscular injection on a horse from the internet, I had an equine vet demonstrate the location of the landmarks on a horse, and demonstrate and aid me with the best technique. He also stayed to critique as I administered my first horse injection. Done properly, the horse simply doesn't care. However, great attention must be paid because even many injections later, a simple movement on the horses part, or even a little speediness or reticence on my own part, can make the injection more dangerous.
Although allergic reactions to immunizations which are common to many species are rare in horses, it does make sense to watch the horse for a few minutes after the injection. It also makes sense to record where the injection was given. For example, if I gave a combination immunization with Eastern, Western and Venezuelan encephalitis on the right side of the horse, and I gave Potomac Horse Fever immunization on the left, and now there is some swelling in the vicinity of the left, then it would be good to know that the horse may be sensitive to that particular immunization.
In our region the immunizations most often recommended by equine veterinarians are:
Eastern and Western and Venezuelan Equine Encephalitis
Influenza and Pneumonitis
West Nile Virus Vaccine
Potomac Horse Fever (Ehrlichia Risticci)
Rabies (which must be annual, not every three years as may be done with dogs and cats)
Most of the time, horse owners buy immunizations which come in combination so as to decrease the number of injections the horse must receive. Some injections are given by themselves. Some injections must be given twice a few months apart before administering either every six months or annually. The advice on an equine vet who knows what the "public health situation for horses" is in your area in invaluable.
Leptospirosis vaccine is not being recommended for horses in my region at this time.
If fed properly, kept in a clean area, permitted adequate exercise and clean water and immunized regularly, most horses can attain 25-30 years of age or more.