Background: The 2002 Recommended Childhood Immunization Schedule clarified the definition of an invalid dose of vaccine as any dose administered >/=5 days before the minimum age or interval had elapsed. Any invalid dose of vaccine should be repeated.
Objective: Determine the proportion of U.S. children who received an invalid dose of vaccine, evaluate the impact on vaccination coverage levels if invalid doses were not counted, and determine the vaccine purchase cost if at least one invalid dose is repeated.
Methods: Provider-reported vaccination histories of children aged 19 to 35 months sampled by the 2000 National Immunization Survey were evaluated. Analyses were performed in 2002 after the 2002 Advisory Committee on Immunization Practices (ACIP) Recommended Immunization Schedule was released. Any vaccine dose administered >/=5 days before the recommended minimum age or interval was classified as invalid. Change in vaccination coverage was determined by subtracting estimated valid-dose coverage (based on number of valid doses received) from the estimated up-to-date coverage (based on number of doses received regardless of age or spacing).
Results: Overall, 10.5% (+/-0.6%) of children had received at least one invalid dose of vaccine. Of the invalid doses, 51% were hepatitis B, 100% of which were the third dose; 19% were diphtheria-tetanus-pertussis (DTP/DTaP), 92% of which were the fourth dose; 12% were measles-containing vaccine (MCV); 15% were varicella vaccine; and 4% were polio vaccine, 96% of which were the first dose. Excluding invalid doses resulted in a small change in vaccination coverage: 2.2% for DTP/DTaP, 0.7% for polio, 6.5% for hepatitis B, 1.4% for MCV, and 1.7% for varicella. The vaccine purchase cost to repeat at least one invalid dose ranged from approximately $10 million (public-purchased) to approximately $18 million (private-purchased).
Conclusions: Nationally about 595,000 of children aged 19 to 35 months, born between February 1997 and May 1999, received at least one invalid dose of vaccine. The cost of revaccinating these children is substantial and may have a negative impact on parents, physicians, and vaccine purchasers. Educating immunization providers regarding proper immunization timing should be conducted to reduce the administration of invalid doses of vaccines.