Vaccinating Your Pregnant Mare
A broodmare's vaccination schedules are somewhat different than other horses. They should be vaccinated at 5, 7 and 9 months of gestation against equine herpesvirus type 1 (EHV-1), which can cause late-term abortions. A killed virus product is usually used for this purpose, and there are vaccines specifically made for pregnant mares.
Broodmares should also receive their yearly vaccinations one month before they are due to foal. This gives them time to develop a strong immune reaction (antibodies) in response to the vaccines. These antibodies are then transferred to the foal in the first milk, called colostrum; this is important because there is no transfer of antibodies across the placenta in mares.
If you vaccinate your horses against equine viral arteritis (EVA), please discuss the timing of this vaccine for broodmares and proper vaccine handling with your veterinarian.