Pups and kittens require vaccinations every four weeks starting at eight weeks up to 20 weeks of age. The answer is somewhat complicated involving the phenomenon of maternal antibody interference with vaccine protection. Newborns receive antibodies from their mother first milk(collostrum) that block the effect of vaccines for a period of time. In other words, pups & kittens receive disease protection through their mother’s milk immediately after birth, especially if mother is current on vaccinations. This protection fades and disappears at some time between 8 and 16 weeks. As long as mother’s protection persists, any vaccinations are eliminated from the baby’s bloodstream by mother’s antibodies and ineffective.
The problem is in knowing the exact time when mother’s antibodies have faded leaving the pup/kit unprotected, needing and able to retain vaccination protection. The only way to know when vaccinations will offer long term protection (8, 12, or 16 or 20week) is to run expensive “antibody titers.” The risk of infection increases exponentially from 8 to 16 weeks. Therefore, repeated boosters are required to ensure the protection.