title: Assessing misclassification of vaccination status: Implications for studies of the safety of the childhood immunization schedule.
abstract: 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.
  
  
  
  To assess risk of misclassification of vaccination status within the Vaccine Safety Datalink (VSD).
  
  
  
  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.
  
  
  
  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.
  
  
  
  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.
authors.affiliation: Institute for Health Research, Kaiser Permanente Colorado, Denver, CO, United States; Department of Pediatrics, University of Colorado School of Medicine, Aurora, CO, United States. Electronic address: matthew.f.daley@kp.org.
authors.forename: Matthew F
authors.initials: MF
authors.lastname: Daley
authors.affiliation: Institute for Health Research, Kaiser Permanente Colorado, Denver, CO, United States. Electronic address: jason.m.glanz@kp.org.
authors.forename: Jason M
authors.initials: JM
authors.lastname: Glanz
authors.affiliation: Institute for Health Research, Kaiser Permanente Colorado, Denver, CO, United States. Electronic address: sophia.r.newcomer@kp.org.
authors.forename: Sophia R
authors.initials: SR
authors.lastname: Newcomer
authors.affiliation: Group Health Research Institute, Seattle, WA, United States. Electronic address: jackson.ml@ghc.org.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Jackson
authors.affiliation: Kaiser Permanente Northwest, Portland, OR, United States. Electronic address: holly.c.groom@kpchr.org.
authors.forename: Holly C
authors.initials: HC
authors.lastname: Groom
authors.affiliation: Kaiser Permanente Southern California, Pasadena, CA, United States. Electronic address: marlene.m.lugg@kp.org.
authors.forename: Marlene M
authors.initials: MM
authors.lastname: Lugg
authors.affiliation: Marshfield Clinic Research Foundation, Marshfield, WI, United States. Electronic address: mclean.huong@marshfieldclinic.org.
authors.forename: Huong Q
authors.initials: HQ
authors.lastname: McLean
authors.affiliation: Kaiser Permanente Northern California, Oakland, CA, United States. Electronic address: nicola.klein@kp.org.
authors.forename: Nicola P
authors.initials: NP
authors.lastname: Klein
authors.affiliation: Immunization Safety Office, Division of Healthcare Quality Promotion, National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA, United States. Electronic address: eiw8@cdc.gov.
authors.forename: Eric S
authors.initials: ES
authors.lastname: Weintraub
authors.affiliation: Immunization Safety Office, Division of Healthcare Quality Promotion, National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA, United States. Electronic address: mmm2@cdc.gov.
authors.forename: Michael M
authors.initials: MM
authors.lastname: McNeil
mesh_terms: D002675:Child, Preschool; D005260:Female; D006801:Humans; D007115:Immunization Schedule; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D012189:Retrospective Studies; D011795:Surveys and Questionnaires; D014611:Vaccination / Q000009:adverse effects* / Q000706:statistics & numerical data*; D000073887:Vaccination Coverage; D014612:Vaccines / Q000008:administration & dosage* / Q000009:adverse effects*
chemical_list: D014612:Vaccines
keywords: Child; Immunization; Under-vaccination; Vaccine; Vaccine safety; Vaccine schedule