I understand that some proportion of individuals will fail to develop immunity from a single dose of a vaccine. Rather than test every member of the target population following each vaccination, it is more economical to administer multiple doses to increase the population immunity rate.
But what determines how many doses are indicated for a particular vaccine, and how widely they are spaced? Are these variables rigorously optimized, or are they just accepted heuristics?
E.g., two doses of MMR are indicated, separated by at least 3 years. Three doses of HPV are indicated, separated by 2 and then 3 months. Four doses of IPV are recommended, as are five doses of DTaP!