Keeping your kitty vaccinated is really important. Even kitties who don't go outside or have visitors should still be vaccinated to protect them against many diseases. The problem is, cats have a weird response to shots. The most worrying is that some cats will develop a fast growing cancer at the shot site. So it is important to keep kitty up to date on vaccinations, with as few actual injections as possible.
Popular belief up until recently was that you needed to run a series of 3 vaccinations close together (3-4 weeks apart) for the most common kitty vaccinations starting when the kitten was young, and then boost again at one year, and then every year after. And for rabies, you needed two shots not too far apart, and yearly boosters. Research has shown, however, that this practice over does it--that cats retain more immunity than previously thought.
Early kitten shots need to be evaluated by each situation. The initial series in kittens is hard because of the immunities they gain from their mother at birth and by nursing, and the timing for those wearing off--it varies and is not entirely predictable. Until those immunities fade, vaccinations will not be effective. So often the first shot will be ineffective, because of immunities still preset in the kitten's system. However, if the kittens wean early, and they will be homeward bound early--that shot can be critical for many. Because of my conditions, environment, and practices I can wait a little longer to give the first kitten shot, and as such, get by fine with only doing two of them. After that, they need another after one year, and then at 3 year intervals.
Now it is possible to fine tune vaccination schedules even further, by the use of titre testing. Unfortunately, not all vets offer this testing, and some will charge through the nose for it--although others are very reasonable. Titre testing uses a sample of your cat's blood to check for antibody levels to the things we vaccinate against. By regularly checking these levels (most commonly at 3 years) you can know exactly when and what your kitty needs to stay protected, and reduce the number of injections (and the amount of adjuvant, antigen, and preservatives--all foreign bodies you are injecting in to your cat)--thereby reducing the risks of injection site carcinoma greatly.