My vet recommend vaccinations every three years for adult dogs. Three-year vaccinations are exactly the same as annual ones (in regards to what’s in them.) In my not-so-professional opinion (though I have done tons of research on this issue), I believe that once immunity to a disease is formed, that immunity exists for life. Unfortunately, lots of initial puppy vaccinations are given when the mother’s antibodies are still in the puppy’s blood, which will kill the disease in the vaccine without allowing the dog to form its own immunity. This is why puppy vaccinations are given (usually) three times in a span of a month or so–it’s basically a guessing game, and they’re guessing when the maternal antibodies will stop being effective.