Validation of the 5-year tetanus- diphtheria- pertussis and polio booster vaccination in the Danish childhood vaccination database.

BACKGROUND: In Denmark- data from the childhood vaccination database are used to calculate vaccination coverage (VC) for childhood vaccinations. However- there may be under-reporting in this database. Accurate VC estimates are necessary for adjusting vaccination strategies and providing population-level protection. AIMS: The main purpose of this study was to validate the reporting of the tetanus- diphtheria- pertussis and polio (Tdap-IPV) booster in the childhood vaccination database- identify reasons a child was not vaccinated- for the unregistered vaccinations- identify where the vaccination was provided- and to adjust calculations of the VC accordingly. METHODS: Children registered in the Danish Civil Registry System (residing legally in Denmark) from the 2000 to 2003 birth cohorts without a recorded Tdap-IPV booster in the childhood vaccination database were randomly selected for this cross-sectional- questionnaire-based study. The adjusted VC in the population was calculated by adding the fraction of the study population registered with the Tdap-IPV booster in the childhood vaccination database to the fraction of the study population who reported being vaccinated on the questionnaire but who were not register according to the childhood vaccination database. FINDINGS: Of the 574 contacted parents- 386 (67%) completed a questionnaire/ 272 (70%) reported that their child received the Tdap-IPV booster- with 121 (44%) providing the date of vaccination. Most commonly reported reasons for not receiving the booster included forgetting (37%) and not wanting the vaccination (16%). The majority (89%) of children who received the booster were vaccinated by their general practitioners (GPs)/ 6% abroad and <1% in a hospital. Using a conservative approach- considering only those who used a vaccination card to answer the questionnaire and who provided an exact data of vaccination- the adjusted Tdap-IPV booster VC was 85.6% (95% CI- 85.1-86.3%) compared to 82% from the childhood vaccination database. CONCLUSION: We identified substantial underreporting of the Tdap-IPV booster in the childhood vaccination database- mainly due to GPs not registering given vaccinations. Validating data used for VC calculations is needed to obtain more precise estimates.