
In the USA all vets have been advised by their ruling body that core vaccinations need only be boosted every third year after the first couse of vaccinatuion and first booster.
The dogs immune system is mature after six months of age and a booster any time after that should confer long lasting or even life long immunity to viruses.
If you do a Google search on Dr Jean Dodds you are bound to come up on her research and advice on this topic. Here is one link:
http://www.doglogic.com/vaccination.htm remember this is America so there are other things that they vaccinate for which we don't.