What Contributed to the Shifts in State Rankings of Vaccination Coverage in 2007 Compared to 2006?

Background:
The CDC releases an annual report on vaccination coverage at national, state and local levels. This year's report showed significant shifts in state rankings compared to the previous year. For the first time in five years, Massachusetts was not listed among the top five states with the highest vaccination coverage, while Maryland, having ranked below the top 10 in previous years, had the highest coverage in 2007.

Objectives:
Assess whether differences in state characteristics contributed to the shifts in state rankings in 2007 compared to 2006.

Methods:
The 2007 National Immunization Survey (NIS) data were used to calculate coverage estimates for the 50 states and the District of Columbia. States were ranked from highest to lowest coverage with the 4:3:1:3:3:1 vaccine series. Direct standardization was used to control for demographic differences across states.

Results:
In 2007, 4:3:1:3:3:1 vaccine series coverage ranged from 91.3% in Maryland to 63.1% in Nevada. Coverage in Maryland increased significantly by 13 percentage points compared to 2006. When adjusted for state differences in child's race/ethnicity, poverty status, maternal marital status and education, and provider type, Maryland continued to show the highest coverage among children aged 19 to 35 months. The order of the remaining top 4 states was unchanged except when adjusted by race/ethnicity differences across states.

Conclusions:
Sampling variability is a known, major contributor to annual shifts in state vaccination coverage rankings; however, demographic shifts do not appear to be the cause of the 2007 shifts. When using NIS data, state immunization programs should focus on potential changes in their states' vaccination coverage over time. Further evaluation of the shift in state rankings in 2007 for the 4:3:1:3:3:1 series will include an assessment of the potential effects of other factors such as differential response rates, size of households with landline telephones, and shifts in individual vaccine uptake across states.