There is ever-increasing controversy as to the necessity and efficacy of yearly or bi-annual vaccinations. I do “spring shots” every year with a back up in fall if needed (EEE is becoming a watch issue, here in New England at least).The show circuit requires much more than the once a year thing, understandably.
There MIGHT be side effects, isolated and so far, manageable, but that is the risk of anything we infuse/inject/ingest. I might go so far as to offer that the side effects are not as detrimental as the diseseas we vaccinate against. Delivery of vaccines, quality of vaccines, lifestyles and the environmental influences can all be considered in whether to vaccinate or not.
I know, around here, there is a very low incidence of Rabies, 1 in 10K(don’t quote me) horses might succumb but 10K or 100K, I don’t want mine to be The One. Yearly vacs seem to be becoming a personal thing for many, I am not sure there is a right or wrong answer any more. Do what is right for your horses and where they live, what they are exposed to, what they do for a living and where, talk to your vet and smile/nod at the rest : )
Sure there's right and wrong but mostly there's just a whole lotta different.