The theory behind vaccinating any pet is to provide artificial exposure to certain disease-causing organisms, thereby priming the body’s immune system before actual exposure occurs.
Doing so will allow for a rapid, effective immune response if this exposure does happen, without the lag time associated with a first exposure.
If the mother has been properly vaccinated prior to pregnancy, most kittens receive protective antibodies from their mother through nursing, primarily during the first 24 hours of life.
These “passive” antibodies are important, since the immune system of a neonate less than 6 weeks of age is incapable of mounting an effective response to any antigen (foreign organism or substance). Around 8 weeks of age, levels of these antibodies begin to taper off, leaving the pet to fend for itself.
If a kitten that still has adequate levels of passive antibodies present in its system is immunized, the vaccination will be rendered ineffective. For this reason, initial vaccinations are usually given around 8 weeks of age, when levels of passive antibodies begin to decrease.
Vaccination as early as 6 weeks of age may be indicated in those instances where the mother has not been vaccinated, or if lack of passive antibody absorption is a possibility (i.e., inadequate nursing during the first hours of life).