A challenge facing immunization registries is developing measures of childhood immunization coverage that contain more information for setting policy than present vaccine series up-to-date (UTD) rates. This study combined milestone analysis with provider encounter data to determine when children either do not receive indicated immunizations during medical encounters or fail to visit providers. Milestone analysis measures immunization status at key times between birth and age 2, when recommended immunizations first become late. The immunization status of a large population of children in the Oregon ALERT immunization registry and in the Oregon Health Plan was tracked across milestone ages. Findings indicate that the majority of children went back and forth with regard to having complete age-appropriate immunizations over time. We also found that immunization UTD rates when used alone are biased towards relating non-UTD status to a lack of visits to providers, instead of to provider visits on which recommended immunizations are not given.

Infants who start receiving immunizations on time are more likely to be up to date at age 2 years. Among 39708 infants aged 3 months covered by the Oregon Health Plan (expanded Medicaid), those who did not have health care coverage within the first month of life were less likely to start receiving immunizations on time. Also at risk were infants in foster care, in subadoptive care, who were blind or disabled, who were Native American or Black, or whose mothers were not covered by the Oregon Health Plan.