- 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.