Vaccines have an important role in preventing diseases in many species, including horses. They also have had an important role on eradicating disease. If you think back to the Equine Influenza outbreak in 2007 vaccination had an important role in preventing spread of disease and eradicating the disease from Australia.
Vaccines are medications which contain either a small portion of the causative agent of the disease that it is designed to prevent (for example the Hendra Virus vaccine contains a small portion of the Hendra virus called the G glycoprotein), or an inactivated form of the causative agent of the disease it is designed to prevent. When the vaccine is administered the horse’s immune system responds by making antibodies against the disease. In the event a vaccinated horse is exposed to the disease it should be able to mount a rapid immune response and not develop the disease. Most vaccines require an initial course of 2 or 3 vaccinations followed by periodic booster vaccinations. It is important to realize vaccination does not cause immediate protection against a disease, so vaccination after exposure to a disease does not mean your horse will be protected against the disease.