BACKGROUND:
The National Immunization Survey (NIS) provides annual estimates of vaccination coverage among children aged 19-35 months for each of the 50 states and 28 selected urban areas. In contrast, immunization coverage is commonly measured by others at 24 months of age, including the Health Plan Employer Data and Information Set (HEDIS®) and many state, county, and local surveys.

OBJECTIVE:
We compare three measures of immunization coverage using data from the 2002 NIS. We compared coverage measured 1) at time of interview in which the child was between 19-35 months, 2) at 24 months of age, excluding those younger than 24 months at time of interview, i.e. the complete-case analysis, and 3) at 24 months of age, using multiple imputation to impute coverage for those younger than 24 months.

METHOD:
We compared, nationally and by selected demographics, coverage with the 4:3:1:3:3 series (at least 4 doses of diphtheria and tetanus toxoids and pertussis vaccine, 3 doses of poliovirus vaccine, 1 dose of any measles-containing vaccine, 3 doses of Haemophilus influenzae type b vaccine, and 3 doses of hepatitis B vaccine).

RESULT:
Both 24-month coverage measures were slightly lower than 19-35 month coverage nationally (72.9, 72.7, and 74.8, respectively), and by most demographic categories. The two types of 24-month coverage estimates were similar. The estimates produced using multiple imputation analyses had narrower confidence intervals than the complete-case estimates.

CONCLUSION:
The three coverage measures were similar enough at the national level for, perhaps in some settings, one to serve as a surrogate for another; larger differences were found among the measures at the state level. Use of multiple imputation resulted in more precise estimates than the complete-case analysis.

LEARNING OBJECTIVES:
To describe the differences between immunization coverage measured using three methods.