To understand the timing and frequency of core vaccines we need to look at several smaller issues and understand the meaning of an important term: maternal antibodies.
These are immune molecules that are transferred from mothers and protect babies during the time their immune system is maturing. A very small amount of these antibodies pass across the placenta while cats and dogs are still in utero, but transfer mainly occurs through uptake into the bloodstream from the intestine when an animal ingests colostrum during the first 24 hours of life. The intestinal tract is only permeable during approximately the first 24 hours after birth, which is part of why nursing is so critical during this early time.
However, the actual amount of protection conferred depends on the mother’s vaccine history and prior natural exposure to disease. In the shelter environment, we rarely know much about either, so we don’t really know if maternal antibody is sufficient in any individual animal. Maternal antibodies also steadily decline over time and have a finite lifespan—usually over in a few months.
Unfortunately, during the time maternal antibodies are present, they inhibit vaccine effectiveness. There is no current vaccine formulation that is fully effective in the presence of maternal antibodies. This ability to essentially block a vaccine from working is what is known as maternal antibody interference.
In shelter medicine, a practical strategy used to overcome the interference of maternal antibody is to begin vaccinating at the earliest time possible for successful and safe vaccination. This is approximately between 4-6 weeks of age. However, in many animals, maternal antibody does not wane enough for a vaccine to be effective until 18-20 weeks. This varies from animal to animal, even within a litter.
Why We Vaccinate So Often
When we talk about maternal antibody interference, we use the term “window of susceptibility.” This refers to the period of time an animal will be unprotected against disease, because maternal antibody levels are too low to protect from natural infection, but too high for vaccination to be successful. This “window” occurs at different times in every individual animal.
We vaccinate as frequently as possible (every 2-3 weeks) in what is essentially a blindfolded race to catch the edge of that window at the earliest time and youngest age possible. This is a race between immunity and disease, and we are trying to vaccinate that puppy or kitten as soon as their maternal antibody level has dropped low enough that their immune system will be able to respond to the vaccine, thereby minimizing the amount of time they are unprotected against natural infection.
The issue isn’t how many vaccines the animal receives—it’s that one vaccine is provided during the exact time the immune system can be stimulated. The older an animal is, the better the immunity, because there are less maternal antibodies interfering and a more mature immune system to respond.
In much the same way that maternal antibody interferes with the ability of a vaccine to be effective, vaccines actually interfere with each other! Giving vaccines closer together than every 2 weeks is actually likely to result in less immunity, not more. Every 2-3 weeks is the optimal frequency.
Is Younger Better?
Most vaccines are actually labeled for use in animals older than 6 weeks, which means they went through formal testing to prove efficacy and safety only in animals of this age or older. We vaccinate shelter animals younger and more frequently than what is typically recommended for pets or by the manufacturer in the label instructions because of the high risk of exposure.
However, vaccinating earlier than 4 weeks of age is less likely to be effective because of the immaturity of the immune system. In addition, for some diseases—most notably canine parvovirus and feline panleukopenia—vaccination at a very young age does have the potential to do harm, particularly to developing brains. Aging puppies and kittens is not an exact science, and when we don’t know their full history or exact age the recommendation to start between 4-6 weeks of age is a safe guideline.