Owners of new puppies and kittens often wonder why their pets need booster vaccines in their first year of life. Even though getting your puppy or kitten fully vaccinated may involve several trips to the veterinarian's office, these boosters are vitally important to your pet's health, and the can prevent life-threatening illnesses in the future.

A vaccine is a weakened or killed form of the virus that causes the disease. The puppy or kitten's body recognizes the virus as something that is foreign, and responds by making antibodies to fight it off. Puppies and kittens normally receive some natural immunity to diseases from their mothers through antibodies in the mother's milk, which are known as maternal antibodies. These antibodies will protect young animals in the first several weeks of life. However, over time, the maternal antibodies will start to "wear off," and the puppy or kitten will then need to start making their own antibodies; this is why we give vaccines. In young animals, the maternal antibodies cannot tell if a virus is from a vaccine or one that will cause illness. Therefore, not only will the maternal antibodies protect the puppy or kitten against disease, it also fights off vaccines so the body can't respond to them.

It would seem the solution would be to just wait until the maternal antibodies wear off, and then start giving the vaccines. The problem is, there is no way to tell when the maternal antibodies start to wear off, because it is different in every animal. In studies, some animals have been found to start losing their maternal antibodies as early as 5 or 6 weeks of age, while others don't lose them until they are 14 or 16 weeks old.

Veterinarians usually recommend beginning puppy and kitten vaccines at 6-8 weeks of age, and giving boosters approximately every 3 weeks until 16-20 weeks of age. This way, no matter when that pet's maternal antibodies wear off, they should be protected.

Occasionally, we will see a puppy or kitten that is partially through their vaccine series become ill, most commonly parvo virus in puppies. Their owners will wonder why their pet got sick even though they had received some vaccines against the disease. This is usually the result of the maternal antibodies again. It is possible for the maternal antibody level to be high enough to keep the vaccines from working, yet not quite high enough to fight off the disease itself. If the puppy or kitten is unfortunate enough to get exposed to the disease during this window of time, they may become ill. This is why we recommend that young animals not be exposed to many other animals, especially those that are sick, until they have completed their vaccine series.