Why do puppies require more frequent vaccinations that older dogs?

Nursing puppies ingest immunizing antibodies from their mother. These maternal antibodies provide early protection against infectious disease. However, they also neutralize the immunizing agents in vaccines. Maternal antibodies naturally decline during the first 3 to 4 months of life and eventually disappear. For this reason, puppies vaccinated earlier than 12 weeks of age. This increases the likelihood of long-term protection from vaccination as soon as maternal antibody levels have declined below protective levels.