Immunity
By Mimi Cary (as posted to the Wellpet list)

The incidence of inadequate immune response to vaccine stimulation is very low, otherwise we humans all will be revaccinating ourselves annually for a multitude of diseases, on a "just in case" basis. The requirement to booster vaccinations in humans as adults is rare, except when specific exposures has occurred (e.g. stepped on a nail) or exposure to an epidemic is anticipated (e.g. cholera).
In any event, if an individual is immune-compromised (e.g. AIDS patient) and therefore has lost immunity, whether systemic-humoral or local-cellular, not only will vaccination be inconsequential, but also, the vaccine itself may become an overburden to the defective immune system. (Imagine pouring oil into a container with a hole in the bottom, trying to fill the container to the "full" line - you can't succeed, and will have made a mess of the surroundings with the leaking oil!) You can also draw your own parallel and apply this to individual animals - dogs, cats, others.
The difference between human and animal vaccines is that human vaccines are more thoroughly researched as required by law. In terms of data on duration of immunity, information is not readily available to the animal practitioner (the data exists, lives in drawers, or in dust on shelves). The time is almost come when duration of immunity information will be on label, and in this manner, the animal practitioner will no longer be obligated to perform unnecessary annual vaccinations on a "just in case" basis, or to avoid potential liability issues.

With My Professional Cap On As I Speak,
Mimi Cary, PhD Biochemistry (Immunology)