I don't know if we're talking about the same vet, but I work for one - and they also are recommending, based on a statement released by the state, that horses get boostered a second time this year if they have not had their WNV booster since May. This may only be a one-time recommendation, so don't necessarily plan on doing it every year - they're just trying to keep on top of the serious mosquito over-population this year. The state just released that statement recently, and I got a postcard in the mail from my vet yesterday saying something to that effect - so if you have horses that haven't been boostered since May, considering doing so again!