Another word for vaccination is immunization. Vaccination works by priming the body's immune system and making it more effective. This is done by injecting the disease for which immunization is sought into the body. That sounds crazy but if the injected pathogen (bacteria or viruses) is live but weakened or dead then they do not make you get the disease but simply trigger your immune system to make it more ready and able to deal with the infection should it come along.
Sometimes however a negative reaction can take place. This after all should not surprise us as kittens are quite fragile and we are injecting them with a virus!
I don't know of the numbers of cat vaccinations gone wrong but they are probably low but significant enough to take note of. Cats can get cancer from vaccines or simply catch the disease from the vaccine. Vaccines can induce life long health problems and kill. These are not that uncommon. A lot of discussion has taken place as to where to inject the vaccination to minimize cancer in the area or minimize the danger from vaccine induced cancer. I am using the word "can" here as clearly most often the benefits of cat vaccinations far outway the detriments.
Why are cat vaccinations routinely given then? Vets recommend a yearly booster. Well if I was being a bit cynical, I'd say a factor was money, the pharmaceutical companies and bread and butter income for the veterinarian surgeries . Vets make a lot of their income from vaccinations and they are in business. When you mix health with the need for profit you don't always get the the right answers.
Most cat keepers follow the vets advice as they don't know better - fair enough. The next time your at the vets ask him/her how many bad reactions cats have to vaccinations. I'd also stay around the vets surgery after a vaccination to make sure all is well. If not you can go back quickly.
In my opinion an older cat doesn't need boosters if she has had vaccinations for many years.
Here is some more on cat vaccination recommendations.
- Wiki (on vaccinations)
- Breeders (on negative effects)
- Myself from general knowledge