The purpose of this study is to determine if a collaborative centralized population-based reminder/recall intervention is more effective than a traditional practice-based reminder/recall intervention at increasing immunizations among young children.

The study proposed is of immense importance in directing future methodology for bringing children who have not received needed immunizations up to date. The proposed study builds on the success of a previous trial and incorporates data from providers and patients to further improve the intervention. In this study the investigators aim to increase the effectiveness, cost effectiveness, and acceptability to practices and families of population-based R/R by introducing a strong private-public collaboration and maximizing health information technologies that allow practices to interface with Colorado's Immunization Services System (CIIS). Two real-world approaches will be compared: one approach will provide primary care practices the tools to conduct recall and educational interventions. In the other approach, population-based centralized recall by the state immunization registry will be conducted in collaboration with practices. Determining which of these methods is most effective in reaching the most children and comparing the cost of each will provide data critical in guiding future national efforts to assure that children entering school are fully vaccinated. Because the trial will be conducted at the level of both urban and rural counties throughout the state and will include all types of providers, the investigators expect our findings to be nationally generalizable.

Inclusion Criteria:

• 19-35 months at time of recall
• has an address in a specified study county listed in the state immunization registry
• child is in need of at least one immunization at time of study

Exclusion Criteria:

• Opted out of the Colorado Immunization Information System (CIIS)
• child is up-to-date on all immunizations