This past Autumn it was time for 3 year boosters for Porter, 7 years, and Becca, 6 years. I chose titers because I expected that they would be fine as they'd had there puppy shots, 1 year booster and parvovirus/distemper 3 yrs later at age 4 yrs. Porter's immunity was great and I will probably not vaccinate or titer him again other than rabies required by law.
Becca, however, didn't show immunity to distemper. The vet gave her a 3 yr booster for parvo/distemper. I was rather surprised that Becca didn't show immunity and I will check her again this Fall to see if her titers show that she now has immunity.
It is easy to just assume that the vaccines work but it may not be the case for all dogs.
Which is part of the reason I don't mind my vet wanting to titer yearly.
However, there are several different possibilities. From what I have been told from immunologists, a negative titer does not necessarily mean a lack of immunity. The animal (or person) can still be immune, but the markers for immune response haven't been triggered recently enough to be measured. In this instance, the animal (or human) would still be able to immediately be able to mount an immune response in the presemnce of active disease.
Or... The vaccine may have a limited ability to protect against disease. These studies have been done on some diseases in people, rarely in animals. My sons were among the first children to recieve the chicken pox vaccine, because they had bad asthma as small children, which meant that chicken pox would have been very dangerous for them. In the beginning, no one knew how long the vaccine would be protective. Sure enough, when my older son was in high school, he got a VERY mild case of chicken pox. Although he really wasn't sick, he was contageous, and had to be out of school for 10 days... A big deal in high school. Now they know that kids do need a chicken pox booster for immunity in adolescence.
The other possibility is that the animal is a true non-responder, who will never maintain a solid immune response. (as a result of a compromised immune system) This is a very small percentage of animals. In that case, according to Dr. Dodds, it doesn't matter how many times, or how often you vaccinate, you still won't protect the animal. That sort of animal needs to be carefully protected from contact with these dangerous diseases.
The problem is, when the dog doesn't have a titer, you don't know for sure which category they are in. So vets tend to err on the side of caution and vaccinate again.