Why Do Baby Animals Need a Series of Shots and How Many do They Need?

When a baby kitten or puppy is born, its immune system is not yet mature; the baby is wide open for infection. Fortunately, nature has a system of protection. The mother produces a certain kind of milk in the first few days after giving birth. This milk is called colostrum and is rich in all the antibodies that the mother has to offer. As the babies drink this milk, they will be taking in their mother’s immunity. After the first couple of days, regular milk is produced and the baby’s intestines undergo what is called closure, which means they are no longer able to take externally produced antibodies into their systems. These first two days are critical to determining what kind of immunity the baby will receive until its own system can take over.

How long this maternal antibody lasts in a given puppy or kitten is totally individual. It can depend on the birth order of the babies, how well they nursed, and a number of other factors. Maternal antibodies against different diseases wear off after different times. We DO know that by 14-20 weeks of age, maternal antibodies are gone and the baby must be able continue on its own immune system.

While maternal immunity is in the puppy’s system, any vaccines given will be inactivated. Vaccines will not be able to “take” until maternal antibody has sufficiently dropped. Puppies and kittens receive a series of vaccines ending at a time when we know the baby’s own immune system should be able to respond. We could simply wait until the baby is old enough to definitely respond, as we do with the rabies vaccination, but this could leave a large window of vulnerability if the maternal antibody wanes early. To give babies the best chance of responding to vaccination, we vaccinate intermittently (usually every 2-4 weeks) during this period, in hope of gaining some early protection.

When a vaccine against a specific disease is started for the first time, even in an adult animal, it is best to give at least two vaccinations. This is because the second vaccination will produce a much greater (logarithmically greater) response if it is following a vaccine given 2-4 weeks prior.

If a Vaccine Lasts a Person for their Whole Life, Why do I Have to Vaccinate my Pet Annually?

In the U.S., vaccines are licensed based on the minimum duration they can be expected to last. It is expensive to test vaccines across an expanse of years so this is not generally done. If a vaccine is licensed by the USDA for annual use, this means it has been tested and found to be protective to at least 80% of the vaccinated animals a year after they have been vaccinated. Some vaccines are licensed for use every three years and have been tested similarly. Do these vaccines last a lifetime? We cannot say that they do without testing and this kind of testing has yet to be performed.

It is also important to realize that some diseases can be prevented through vaccination while others do not. For a vaccine to generate solid long-lasting immunity, the infection must be fairly generalized to the entire body (such as feline distemper or canine parvovirus) rather than localized to one organ system (such as kennel cough or feline upper respiratory viruses). Vaccination for localized infections tends to require more frequent boosting, whereas there is potential for vaccination for systemic disease to last for many years.

Since the mid-1990s most veterinary teaching hospitals have restructured their vaccination policies to increase the duration of some vaccines from one year to three years based on independent studies rather than on the studies used by the USDA for vaccine licensing. The American Association of Feline Practitioners has vaccination guidelines for cats living in different exposure situations, and the American Animal Hospital Association has guidelines for dogs.

It is important to realize that these are just guidelines and different regions and different pet lifestyles will justify modifications.