Foals are born around April and in a few months, they will need to be vaccinated. The question is when to begin. A lot of this depends on the vaccination status of the mare as historically, foals born to vaccinated mares are not vaccinated the same way as young as foals born to unvaccinated mares. Foals born to unvaccinated mares should have it early, beginning at three months of age, as they have no immunity to prevent disease. Foals born to vaccinated mares will have immunity transferred from the mare to the foal at birth through the colostrum, assuming the foal received adequate colostrum. Because of this, foals born to vaccinated mares are routinely vaccinated later because they are assumed to be protected, and it is believed the antibodies from the mare would cancel out any vaccinations given to the foal starting at three months of age.
However, there was a lack of scientific evidence to support this theory and the veterinarians at Kansas State performed a study in which they vaccinated foals with three doses of vaccine beginning at three months of age in one group, and at six months in another group. Foals were vaccinated for encephalomyelitis, tetanus, West Nile, and equine herpes virus. Both groups were again vaccinated at 11 months of age. Results indicated that all foals, regardless of when vaccinations were begun, responded to the vaccines and even foals vaccinated at three months of age are capable of immune activation despite antibodies received from the mare. So if you have foals in a critical situation and need early protection against these diseases, you can begin vaccinating foals at three months of age; if three doses are given followed by a booster at 11 months of age, it is likely the foals will respond to the vaccine.