In the U.S., vaccines are licensed based on the minimum duration they can be expected to last. It is expensive to test vaccines across an expanse of years so this is not generally done. If a vaccine is licensed by the USDA for annual use, this means it has been tested and found to be protective to at least 80% of the vaccinated animals a year after they have been vaccinated. Some vaccines are licensed for use every three years and have been tested similarly.
Do these vaccines last a lifetime?
We cannot say that they do without testing and this kind of testing has yet to be performed.
Some diseases can be prevented through vaccination while others do not
It is also important to realize that some diseases can be prevented through vaccination while others do not. For a vaccine to generate solid long-lasting immunity, the infection must be fairly generalized to the entire body (such as feline distemper or canine parvovirus) rather than localized to one organ system (such as kennel cough or feline upper respiratory viruses). Vaccination for localized infections tends to require more frequent boosting, whereas there is potential for vaccination for systemic disease to last for many years.
Since the mid-1990s most veterinary teaching hospitals have restructured their vaccination policies to increase the duration of some vaccines from one year to three years based on independent studies rather than on the studies used by the USDA for vaccine licensing. The American Association of Feline Practitioners has vaccination guidelines for cats living in different exposure situations, and the American Animal Hospital Association has guidelines for dogs.
It is important to realize that these are just guidelines and different regions and different pet lifestyles will justify modifications.
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