The need for surveillance of delay in age-appropriate immunization

Background: Vaccination status is assessed nationally in terms of up-to-date status without regard to the age at which recommended doses were actually received. Our study was conducted in 2000-2001 using the most current National Health Interview Survey (NHIS) public use files available. Methods: Retrospective analysis to determine up-to-date and age-appropriate vaccination status for children aged 25 to 72 months. Five years of pooled data (1992-1996) were obtained from the NHIS Immunization Supplement for children aged 25 to 72 months with immunization data based on written records. The outcome measures used were months of vaccination delay relative to age-appropriate vaccination standard as well as up-to-date vaccination status for the fourth diphtheria-tetanus-pertussis (DTP 4), Polio3, the first measles-mumps-rubella (MMR1) doses, and the 4:3:1 series. Results: Of the 9223 eligible children, 80% were up-to-date for the 4:3:1 vaccination series, but 48% had experienced delays relative to age-appropriate standards. For the DTP4 dose, 85% were up-to-date, although only 46% had received this dose at the appropriate age. Similarly, 90% of children were up-to-date with their Polio3 dose, with 64% receiving this dose at the appropriate age; 96% were up-to-date for the MMR1, and 58% received this dose at the appropriate age. Age-appropriate DTP4 vaccination increased by 17 percentage points from 1992 to 1996, whereas up-to-date DTP4 status increased by only 6% during the same period. Conclusions: Children with up-to-date vaccination status often experienced considerable delay relative to age-appropriate vaccination standards. Consequently, vaccination status measures based solely on up-to-date status tend to understate the degree of underimmunization in a population. National surveillance of age-appropriate vaccination is necessary to identify subpopulations with the greatest prevalence of vaccination delay and to reveal underlying trends that may not be evident through assessments of up-to-date status.