My miniature horse injured her shoulder last February (it has healed). However, after her first West Nile virus vaccination over a year later, she became lame in that leg/shoulder again. Is this documented as a possible side effect to the vaccination? I am concerned about her receiving the second injection, which is due shortly. She is just now recovering from the first one.

You are correct to be concerned whenever a horse has a possible adverse reaction; future reactions might be more frequent and, indeed, more severe. However, an exacerbation of a previous injury–or for that matter, forelimb lameness–is not a common problem we see with vaccine adverse reactions. It is difficult to be certain that what you have seen is related to vaccination, especially with a history of injury, but it is certain that West Nile virus is a risk to your horse.

Consult with your veterinarian–as you should do with all preventive health measures–to determine if the risk you perceive from vaccination outweighs the benefit of vaccinating against this epidemic, life-threatening disease.