It is important to vaccinate horses when they are young, unvaccinated or have no history of being vaccinated.  Mares that are being bred need to be vaccinated prior to breeding and 4-6 weeks prior to foaling.  Once the foal is born your veterinarian can help

Vaccinations should not be a regimented procedure.  Each animal needs to be addressed as an individual.  The things that need to be evaluated as to age, lifestyle, and potential exposure to animals that have disease, disease in the community and the severity of clinical disease that is prevented by the vaccine.  It is important to remember that vaccination is a medical procedure and does deserve a great deal of thought as would be given to any serious healthcare decision.