BACKGROUND:
Coverage through Medicaid is characterized by gaps in coverage, related in part to frequency of renewal policies. Some states (like Michigan) require renewal every 12 months; others (like Oregon) every 6 months. Studies generally have not explored the effect of state renewal policies on outcomes.

OBJECTIVE:
To determine the effect of enrollment time in Medicaid on up-to-date (UTD) coverage rates and whether frequency of renewal in Medicaid appears to affect coverage.

METHOD:
Data were collected from Medicaid databases and immunization registries in Michigan and Oregon, for children who turned two in 2001. Children were grouped according to length of time in Medicaid: < 6 months; >= 6 but < 9 months; >= 9 months but < 12, and >= 12 months. The Medicaid enrollment files were matched with files from the state immunization registries. Up-to-date coverage (4:3:1 series) was calculated for children overall and for children in each enrollment group.

RESULT:
More children were enrolled for >=12 months in Michigan (85%), with 12-month renewal than Oregon (55%), with six month renewal. Overall UTD coverage rates for two-year-olds were higher in Michigan (62% UTD) than Oregon (56% UTD). Children enrolled for shorter periods of time were less likely to be UTD than children enrolled for 12 months or more in both states, with UTD rates comparable in each enrollment grouping. Children enrolled <=6 months were 44% UTD (Michigan) or 46% UTD (Oregon). Children enrolled >=12 months were 63% UTD (Michigan) and 62% UTD (Oregon). Michigan's higher coverage rates were due to the fact that more children were enrolled for >=12 months.

CONCLUSION:
There appears to be a strong relationship between renewal policies, length of enrollment in Medicaid and UTD immunization coverage rates.

LEARNING OBJECTIVES:
To demonstrate that state renewal policies can affect quality.