When a baby kitten or puppy is born, its immune system is immature, leaving it vulnerable to infection if exposed to diseases. During the first few days of nursing, the puppy or kitten receives mother’s milk called colostrum which is rich in all the antibodies the mother has to offer. How long this maternal antibody lasts in a given pet is very individual and depends on a variety of factors.
What we do know about maternal antibody is that it starts to wane as early as 6 weeks of age, and is completely gone by 16 weeks of age. The puppy or kitten’s body must then depend on its own antibodies for protection. While maternal antibodies are in a puppy or kitten’s body, any vaccine given will be inactivated and will not result in antibody production or protection for the pet.
Because it takes one vaccine and then a booster vaccine 2 – 4 weeks later to provide adequate antibody production in an individual animal, our goal with a series of vaccines is to provide two effective (not inactivated by maternal antibody) vaccines, 2 – 4 weeks apart, during the period of time when the maternal antibody is leaving their body and their own immune system is able to respond. That said, it is not the number of vaccines given in the series that is important (it only takes two), but the period of time over which the vaccine series is given that is important.