Why do they own a West Nile vaccine for horses but not for humans?

Why do they own a West Nile vaccine for horses but not for humans?
Perhaps the equine vaccine was originally designed for human use but cause too many bad effects in humans and be relegated to animal use.
But there is a WNV vaccine surrounded by the works for humans. It is unknown when, if ever this will hit the market.
A horse's natural life is worth more than a man's.
The testing and approval process is more stringent and time consuming for human medication and vaccines than it is for animals.