This information is directly from VCA Hospital Website
Immediately after birth, a kitten receives a temporary form of immunity through the colostrum, which is the milk produced by mother cats shortly after birth, laden with protective antibodies. This first milk is produced only for a few days after birth and contains proteins called maternal antibodies. For about 24-48 hours after birth, the kitten's intestine allows absorption of these antibodies directly into the blood stream. This passive immunity protects the kitten during its first few weeks of life when its immune system is immature, but in order to remain protected against these diseases, the kitten must produce its own, longer-lasting active immunity. Vaccinations stimulate active immunity, but they have to be given at just the right time. As long as the mother's antibodies are present in the kitten’s bloodstream, they prevent the immune system from responding effectively to the vaccines. When a kitten is ready to respond to vaccinations depends on the level of immunity in the mother cat, the amount of antibody absorbed by the nursing kitten, and the general health and nutrition of the kitten.
Since it is difficult to know exactly when an individual kitten will lose its short-term passive immunity and be ready for immunizations, a series of vaccinations given at specific intervals increases the odds of stimulating active immunity in the kitten. The goal is to give at least two vaccinations in the critical window of time that occurs after the kitten loses her maternal immunity and before she is exposed to infectious diseases. Giving a series improves the chances of hitting this window twice. Also, multiple injections are needed because a single vaccination, even if effective, is not likely to stimulate the long-term active immunity that is so important. Rabies vaccine is an exception since one injection given at the proper age is enough to produce lasting immunity.
To keep up the cat’s immunity through adulthood, vaccines are repeated once every 1-3 years depending on individual circumstances and vaccine type.