A model for systems-level influences on toddler immunization completion.

Toddler immunization completion rates vary across populations in the United States, and this variation may be contributing to the national rise in vaccine preventable diseases. Yet existing theoretical scholarship for improving toddler immunization uptake is largely focused on the individual level, neglecting the multiple system-level environments that should be considered. Population characteristics are distinct from the sum of individual characteristics. Thus, a reformulation of the ecological systems theory is proposed to provide theoretical direction and guide future population health research regarding system-level influences on vaccine coverage.