Vaccinations work by stimulating antibodies in the blood to fight off the harmful effects of bacterial or viral infection. Once an animal is exposed to a mild form of a disease through an inoculation, he can better resist infection caused by the disease pathogens.
Horses are more capable of building immunities if they receive vaccinations at a time when they are already healthy and in good condition. An ill, stressed, or undernourished animal may not have the reserves to effectively produce antibodies and, therefore, may not be sufficiently protected if exposed to a contagious disease via vaccination.
Become familiar with the diseases that affect horses in your area. Learn everything you can about the risks of disease versus the risks of vaccination against the disease; sometimes the vaccine is not worth the slight risk that your horse would contract the disease.
It is impossible to provide sound advice regarding the complex nature of diseases that affect horses in the various regions of the country. It is best to work with your veterinarian to develop an effective vaccination program for your horse.