Responsible pet owners know that getting a cat its recommended vaccinations can be one of the most critical factors in preventing serious cat illness. Unfortunately, vaccines have been known to fail in rare instances, for a variety of reasons. Just as it's important for owners to get their cats vaccinated, it's also important they educate themselves on why vaccines fail.
Vaccines essentially introduce a weakened version of the virus to the feline, which may be why the cat may show a few clinical signs of the illness after receiving the shot. The idea is that the cat will generate immunity against the virus through the vaccine. However, there are situations where this may not happen.
For example, VCA Animal Hospitals reports that vaccines have been known to be ineffective in very young kittens that have just recently received natural antibodies from their mothers' milk. This natural immunity essentially overrides the stimulated immunity that the vaccine is intended to provide. This is why it is important to follow your veterinarians recommendations on when to start a kitten’s vaccine series. Also, if a cat has a disease or is on medications that suppress its immune system, the vaccine may also be ineffective.
Other problems may lie with the vaccine itself. The feline leukemia vaccine, for example, is effective in about 80 to 95 percent of cats it is given to - in other words, it's not perfect. Vaccines can also be rendered ineffective if they are warmed during transport rather than chilled. Also the immunity created by some vaccines is short-lived and may not last a full year. It is important to discuss which vaccines are best for your cat with your veterinarian and at what time interval they should be given to provide the best protection against common cat diseases.