Newborn foals have an increased susceptibility to disease because their immune system is limited at this age. Initial immunity is provided through the colostrum or first milk from the mare and if adequate colostrum is not ingested, foals are at risk for serious infections. For long-term immunity to develop, vaccination against disease is required although it is questionable if young foals will respond to vaccine as their immune system is immature. Also, since they receive antibodies from the mare, it is possible these antibodies from the mare will prevent the foals from responding to the vaccine.
To determine if vaccines are effective, a study was performed at the Universities of Georgia and Kentucky in foals that were born of vaccinated mares and foals from non-vaccinated mares. The vaccination of foals from non-vaccinated mares was effective after foals reached 1 month of age. However, the immunity was short lived. Foals vaccinated at an early age that were born from vaccinated mares did not respond well to the vaccine unless a recombinant vaccine was used. Although these foals did not respond to the vaccine initially at a level to prevent disease, the administration of the vaccine did not prevent them from responding later in life as some had suggested. The important result of this study is that the immunity in foals vaccinated early in life is short lived so consequently, it is a good idea to give all the vaccines your veterinarian recommends for your foals. Just giving one or two vaccines in the series is unlikely to prevent your foals from developing diseases.