We don't vaccinate ANY of our pregnant mares for ANYTHING and we've never had any problems with mares aborting.
The only year we did was the first year where everyone was so scared over the West Nile Virus. We vaccinated all of our horses in April-pregnant mares and open mares alike...we only ended up with 4 foals the next year. That's 4 foals from about 25 mares bred! Mares didn't come into heat and we had 8 mares absorb their foals. NO MORE VACCINES FOR OUR PREGGIES!