Comparison of Vaccination Coverage Estimates: National Immunization Survey (NIS) Vs. Immunization Information Systems (IIS)

Background: The National Immunization Survey (NIS), a nationwide, list-assisted random-digit dial survey, monitors the vaccination rates of children between the ages of 19 and 35 months.  Immunization Information Systems (IIS), also known as immunization registries, attempt to collect and consolidate immunization information from multiple providers for all children in a geographic area into one database.

Objectives: Compare coverage estimates in the IIS to estimates in the NIS to assess completeness and accuracy of vaccination information in both sources.

Methods: During the 2008 NIS household interview in nine states/local areas, parents who granted consent to contact providers were also asked for consent to contact the IIS to obtain immunization information. The IIS was provided a list of the consented children and corresponding demographics, for use in finding and extracting each child’s IIS data.  Up-to-date (UTD) rates were calculated using NIS data, IIS data, and the synthesized NIS/IIS combined data.  For sample children who had either adequate NIS provider data or adequate IIS data, synthesized NIS/IIS UTD status was based on combined vaccination histories from NIS and IIS.

Results: Agreement between NIS and IIS UTD status for the 4:3:1:3:3:1 series ranged from 45.4 to 91.8 (median=74.0) percent for the nine study areas.  NIS UTD rates for 4:3:1:3:3:1 were higher than those from the IIS in eight areas, with differences ranging from 4.2 to 45.9 (median=12.8) percentage points.  Synthesized NIS/IIS UTD rates for 4:3:1:3:3:1 were higher than for NIS in eight areas, with differences ranging from 0.1 to 6.8 (median=1.6) percentage points.

Conclusions: While the NIS and the IIS reported the same vaccination status for over half the children, when the two sources differed, the NIS was more likely to report the child to be UTD.  Results indicated that when vaccination data in the IIS were combined with the NIS vaccination data, vaccination rates were somewhat higher overall.