Birth Order and Up-to-Date Vaccination Status

Routine childhood vaccinations are administered at designated ages and intervals according to the Centers for Disease Control and Prevention’s (CDC’s) recommended schedule.1 Although delayed immunizations could expose young children to serious but preventable infectious diseases, late vaccination and refusal to vaccinate remain significant issues.2 Global vaccine coverage may be affected by factors such as family social context, type of health care system, and child-related attributes such as birth order.3 Past literature has returned mixed results on the relationship between birth order and physical/mental health outcomes.4 Although several international studies have found links between birth order and vaccination coverage, there has been no recent, national study on such an association in United States children.5,6 This study compared vaccination up-to-date (UTD) status between first-born and non-first-born United States children using 4 years of data from a nationally representative dataset.The CDC-administered National Immunization Survey–Child (NIS–Child) is a nationally representative survey covering United States children aged from 19 to 35 months. In this cross-sectional study, the 2016–2019 NIS–Child data sets were combined for analysis according to the NIS Data User’s Guide (n = 62 343).7 The 2020 data set was excluded because of overlap with the coronavirus disease 2019 pandemic, which could introduce additional confounders. Logistic regression was used to model likelihood of vaccination UTD status (defined by the CDC and reported by child’s medical provider) as a function of the child’s first-born status as reported by the survey respondent (child’s caregiver/other household member). The vaccines included in this analysis were recommended to be administered before 19 months. Survey-weighted odds ratios, adjusted odds ratios (aORs), and 95% confidence intervals (CIs) were computed, with models controlling for potential sociodemographic confounders such as child’s age, sex, race/ethnicity, mother’s age and level of education, household poverty status, insurance coverage, and number of children aged <18 years in the household. The threshold for significance was P = .05. All analyses were conducted using the survey package (which accounts for the weighted nature of the data set) on R (version 4.1.1). This study was exempt from institutional review board review because of the use of publicly available, de-identified data.The total eligible sample included 24 582 first-born and 37 761 non-first-born children, which accounted for 38.68% and 61.32% of the sample, respectively (weighted). After adjusting for sociodemographic variables, first-born children were significantly more likely to be UTD for all 8 individual vaccines and all 4 vaccine series examined (Table 1). When the number of children aged <18 years in the household was introduced into multivariate models as a control, first-born children were significantly more likely to be UTD for 4 of the 8 individual vaccines. Specifically, they had the highest likelihood of being UTD for pneumococcal conjugate vaccine (aOR = 1.30, 95% CI: 1.13–1.50, P < .001) and diphtheria, tetanus, and acellular pertussis (aOR = 1.29, 95% CI: 1.12–1.48, P < .001), compared with non-first-born children. First-born children were also significantly more likely to be UTD for all vaccination series, including 4:3:1:3 (aOR = 1.19, 95% CI: 1.05–1.35, P = .007), the least strict of all measured series (Table 1). Additional analysis of interaction terms between first-born status and household income showed no statistically significant effect of interaction on likelihood of vaccination.This analysis of a large, nationally representative data set found that first-born United States children were more likely to be fully UTD on all individual routine childhood vaccinations and vaccination series than non-first-born United States children. When the number of children in the household was introduced as a control, the results were attenuated, suggesting that family size may partially, but not fully, explain these results. Although decreased parental investment and constraints on resources have been postulated as underlying factors for the relationship between birth order and health outcomes, future research should seek to illuminate potential explanations for this trend.8 First-born children may be unique for new parents, who may feel a sense of urgency that is no longer felt with non-first-born children. Additionally, parents may try to minimize time off from work by scheduling multiple children’s visits simultaneously, which could result in vaccination schedules not coinciding and series being delayed. Pediatricians should be aware of this possibility and reinforce the importance of on-time vaccination schedules for parents with multiple children. Identification of risk groups for undervaccination is essential for public health and the prevention of severe childhood illness.Limitations of this study include the generally low response rates for NIS telephone surveys (21.1%–32.1% for cellphone, 51.9%–55.7% for landline across studied years) and low percentage of total respondents with adequate provider vaccination data (49%–56% of completed interviews).7 Our analysis also does not address additional factors that could mediate the studied relationship. For instance, rates of participation in well–child visits have been linked to birth order and could be an area for targeted intervention to improve vaccination rates.8 Because delays in vaccination may be exacerbated by the coronavirus disease 2019 pandemic, prompt intervention to address worsening disparities in vaccination coverage is of vital importance.9