title: Birth order and private voluntary immunization--a study of 110,902 children.
abstract: BACKGROUND
  Introduction of new private, voluntary immunizations often results in low vaccine uptake among certain sub-groups within the population. Revealing factors associated with underimmunization is crucial in vaccine endorsement and distribution.
  
  
  OBJECTIVE
  Our goal was to investigate the effect of child's birth order on private voluntary varicella vaccination.
  
  
  METHODS
  A nested case-control study was conducted on a cohort of 110,902 Israeli children under the age of 5 years. We compared social and demographic factors of immunized and unimmunized participants. Logistic regression models were built to examine the association between birth order and vaccination, controlling for child's age, gender, country of birth, ethnicity, parents' country of birth, area of residence, and socioeconomic status (SES).
  
  
  RESULTS
  Ethnicity had the highest association with varicella immunization status. The odds of vaccination in the general Jewish and Ultra-Orthodox Jewish populations were 25.55- (95%CI:20.13;32.42) and 15.04- (95%CI:10.18;22.22) times the odds in Arab population, respectively. Child's birth order was inversely related to vaccination status and presented a nonlinear exposure-response relationship. This relationship was maintained in different ethnicity and SES groups. Child's birth order was associated with vaccination differently in large (> 3 siblings) and small to average-sized sibships (≤ 3 siblings). Other parameters associated with vaccination were child's and parents' country of origin, area of residence and SES.
  
  
  CONCLUSIONS
  Birth order is an independent risk factor for underimmunization, associated with child's vaccination status beyond economic, social, and demographic parental characteristics.
authors.affiliation: Department of Epidemiology and Biostatistics, Ben-Gurion University of Negev, Israel.
authors.forename: Natalie
authors.initials: N
authors.lastname: Gavrielov-Yusim
authors.forename: Erez
authors.initials: E
authors.lastname: Battat
authors.forename: Lily
authors.initials: L
authors.lastname: Neumann
authors.forename: Michael
authors.initials: M
authors.lastname: Friger
authors.forename: Ran D
authors.initials: RD
authors.lastname: Balicer
mesh_terms: D001722:Birth Order; D016022:Case-Control Studies; D019433:Chickenpox Vaccine / Q000008:administration & dosage* / Q000276:immunology; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D007722:Health Knowledge, Attitudes, Practice; D006801:Humans; D007114:Immunization / Q000379:methods* / Q000706:statistics & numerical data*; D007223:Infant; D007231:Infant, Newborn; D007557:Israel; D008297:Male; D015233:Models, Statistical
chemical_list: D019433:Chickenpox Vaccine