Another couple of things that everyone really should remember:
1-- No vaccine is ever 100% safe, nore is it always 100% effective. ANY animal at ANY time can react to it. You vaccinate because in the majority of cases the vaccine will save the animal's life if the animal does contract the illness. So each time you give a vaccine, there is ALWAYS a small risk. Doesn't matter if it is horses or humans.
2-- When was the vaccine given??? One should never give a vaccine during the first trimester as that is the time that most of the birth defects in fetuses happen. This is all too often not said to horse owners, and I know of many who have vaccinated during this time and been ok. But still, I go out of my way to avoid doing any medicating during that critical first trimester.
And there have been some issues with some horses from the WNV vaccine. But then again, I had a year 2 years ago where the standard vaccines caused every horse I own to be lame for over a week! Yep, 100% of my horses reacted and later I found out that it was due to the carrier that the vaccine was suspended in not being quite right. The WNV was not even given at the same time as I wanted to be very sure how my horses reacted to the WNV vaccine. If I had vaccinated for WNV, you can bet that I would have blamed it as I had never used it before. It turned out that there were manufacturing issues with my batch of vaccine and they were all supposed to have been pulled and returned to the manufacturer. So did these people who had supposed issues vaccinate for all things at one time and the issues were really from the regular vaccines and not the WNV vaccine????
Finally, there are some things that really are linked to certain products made by certain company, and people have had to fight tooth and nail to get it noted that there were issues with these products. If the drug company had been more proactive in not discounting the customer's concerns, then many of these claims would not be posted all over the internet. It is funny, but it is pretty much one company that everyone is always complaining about.... Moxidectin comes to mind and not only does it bother minis and mini owners were told there was no basis to make those claims on (I had my mares abort within 24 hours of administering it at proper dosage for my horse's weight!), but their formula for dogs was recalled as it was killing dogs. So who can really blame people for not trusting the original WNV vaccination and blaming it on that vaccination and the company that manufactures it. Doesn't make it right, but well, the track record hasn't been to great for that certain company either.
With that said, I have used that certain company in the past, and will use them again if I can't find another vaccine to use. I feel it is a very personal choice and that I would rather risk the side effects than to leave my horses totally unprotected.