When WNV first became a problem, I was not going to vaccinate. I took my mare in for a preg check, and 4 employees at the vet clinic asked me if I was going to vaccinate, and all, including the vet, said they had had NO reactions. Famous last words!! I vaccinated, and confirmed her in foal. The next day, she came in looking like she had Parkinsons...head shaking, walking like she was blind. Reacting to the vaccine. She lost that foal, and I could not get her settled at all that year.
I do not WNV vaccinate ANY pregnant mare now.