Why do puppies and kittens receive two doses of vaccine?

Young animals generally receive a primary course that consists of two doses, administered a number of weeks apart.

Maternal immunity is conferred on the puppy or kitten in its mother's first milk. This declines over the first few weeks, but while levels remain high this can interfere with the efficacy of the vaccine. Two doses ensure early protection against disease if this acquired maternal immunity is inadequate and also aims to provide a smooth transition from maternal to vaccinal protection. The potential for an "immunity gap" when puppies and kittens could prove susceptible to infection is thereby minimised.

In addition some vaccines actually require a two-dose initial course to produce their maximum immunity. This is particularly true of 'killed' vaccines, such as those for leptospirosis in dogs.