When a puppy or kitten is born its immune system has not yet matured. Kittens and puppies remain protected from infection because of the transfer of maternal antibodies in special milk from their mother called colostrum. Colostrum is only produced a few days after giving birth and it can only be accepted during the first days after birth before their intestines “close up” and can no longer absorb the antibodies.
Sometimes antibodies are not transferred either because the puppy did not drink the colostrum or the mother did not have certain antibodies, other times those transferred only last for a short period of time. If an appropriate number of antibodies are transferred there is protection for a longer period of time. However, the maternal antibodies that are transferred do not last forever.
What does this have to do with vaccinations? Well, there is really no way to tell if maternal antibodies were transferred, which were transferred or how long they will last and therefore a vaccination given early will help protect them.
So why not just give one vaccine early and forget about it? If a vaccine is given to a pet that still has maternal antibodies present it will be inactivated and new antibodies will not be made. How long maternal antibodies last in a pet varies with each individual and for specific diseases. We do know that maternal antibodies are gone or have decreased enough to not interfere with vaccines by 16-20 weeks of age. This is why the final booster is usually given at this time when there will be no interference and protection ensured.
So why not just give one vaccine when they are older, when we are pretty sure maternal antibodies will not interfere with vaccines? Again, since there is no easy way to tell if a pet has protection from maternal antibodies at birth or if protection will last until 16-20 weeks of age, waiting to vaccinate until that time puts them at risk of developing dangerous and deadly diseases.
Therefore, vaccinations are given in series every 2-4 weeks until age 16-20 weeks. This is done to protect those that lose maternal antibodies early while ensuring that those that have longer lasting maternal antibodies are protected as well.
What if the pet is older than 16 weeks of age? In this case there is no worry about interference from maternal antibodies, but one booster 2-4 weeks after the initial vaccine is necessary to ensure a better response.
