Assessing misclassification of vaccination status: Implications for studies of the safety of the childhood immunization schedule

Vaccine. 2017 Apr 4;35(15):1873-1878. doi: 10.1016/j.vaccine.2017.02.058. Epub 2017 Mar 9.

Abstract

Background: To address public concern about the safety of the childhood immunization schedule, the Institute of Medicine recommended observational studies comparing adverse health outcomes of fully vaccinated children to children under-vaccinated due to parental choice. Misclassification of vaccination status could bias such studies.

Objective: To assess risk of misclassification of vaccination status within the Vaccine Safety Datalink (VSD).

Design/methods: A retrospective cohort study was conducted in three phases. In phase 1, electronic health record (EHR) data were used to identify patterns of under-vaccination during the first 24months of life potentially due to parental choice. In phase 2, a random sample of records of under-vaccinated children was manually reviewed. In phase 3, a separate sample of parents were surveyed to assess whether EHR data accurately reflected their child's vaccination status. Phases 1 and 2 were conducted at 6 VSD sites, phase 3 at 1 site.

Results: The study cohort included 361,901 children born 2004 through 2012. By 24months of age, 198,249 (54.8%) were fully vaccinated with no delays, 84,698 (23.4%) experienced delays but were fully vaccinated by 24months of age, 4865 (1.3%) received no vaccines, 3789 (1.0%) delayed starting vaccination until ≥4months of age, 4781 (1.3%) had consistent vaccine-limiting (≤2 vaccines per visit), and the remaining 65,519 (18.1%) were missing vaccine series or doses. When a diagnosis code for vaccine refusal was present in EHR data, encounter notes confirmed vaccine refusal as the reason for under-vaccination for nearly 100% of sampled records. Parent surveys confirmed these findings. Parents of under-vaccinated children were more likely to report visiting an alternative medical provider than parents of fully vaccinated children.

Conclusions: Specific groups of children, under-vaccinated due to parental choice, can be identified with relatively low likelihood of misclassification of vaccination status using EHR-based vaccine data and diagnosis codes.

Keywords: Child; Immunization; Under-vaccination; Vaccine; Vaccine safety; Vaccine schedule.

MeSH terms

  • Child, Preschool
  • Female
  • Humans
  • Immunization Schedule*
  • Infant
  • Infant, Newborn
  • Male
  • Retrospective Studies
  • Surveys and Questionnaires
  • Vaccination / adverse effects*
  • Vaccination / statistics & numerical data*
  • Vaccination Coverage*
  • Vaccines / administration & dosage*
  • Vaccines / adverse effects*

Substances

  • Vaccines