Why are puppies and kittens vaccinated every 3 weeks until they are 16 weeks old?

A puppy or kitten receives antibodies from its mother. The levels of these antibodies will gradually fall (from birth to about 12-15 weeks of age) as the puppy or kitten becomes older. The rate at which these protecting antibodies drop or fall below a level (I'll call it the "critical level") that would make the pet potentially susceptible to infection is variable. If the maternal antibodies are high enough they will block a vaccine from "working." However, once the antibodies fall below the critical level there are not enough maternal antibodies to block the vaccine and the vaccine will be effective.

The catch is that it is almost impossible to know when that critical level for maternal antibodies will fall low enough for a vaccine to be effective. For some pets it may occur at 7 weeks and another at 15 weeks, and others may fall anywhere in between.

For this reason kitten and puppy vaccination schedules are set up on a 3 week cycle until the age of 16 weeks. We are basically trying to ensure that during one of those vaccinations maternal antibodies will be low enough for the vaccination to be protective. If vaccinations are administered too closely together, say every 2 weeks, the body's immune response can interfere with the most recent vaccination.

Tad Squires, DVM
River City Veterinary Hospital
2250 W. Everest Lane
Meridian, ID 83646
http://www.rivercityvet.com/
info@rivercityvet.com