Independent and joint associations of sex and birth order with non‑national immunization program vaccine coverage among Chinese children: a cross-sectional analysis.

Background: In response to declining fertility rates, China implemented universal two- and three-child policies, leading to an increase of multi-child families. However, little is known whether and how these policy changes influenced the uptake of non-National Immunization Program (non-NIP) vaccines. We evaluated associations of sex and birth order with non-NIP vaccine coverage in 1-6-year-old children in China.

Methods: Our study was a cross-sectional survey of caregivers of 1-6-year-old children in Zhejiang and Henan Provinces in 2022. We obtained information on sex, birth order, vaccination history, and family socio-demographics and determined uptake of five commonly-used non-NIP vaccines for infants [Haemophilus influenza b (Hib), varicella, rotavirus, enterovirus 71 (EV71), and 13-valent pneumococcal (PCV13) vaccines]. Children were considered vaccinated if they received the first dose of at least three of these vaccines. Log-binomial regression models were used to estimate prevalence ratios (PRs) and 95% confidence intervals (CIs) for independent and joint associations of sex and birth order with non-NIP vaccine coverage, and multiplicative and additive interactions of sex and birth order.

Results: A total of 1611 children with accompanying parents/guardians were included in the survey; 48.0% (n = 773) were girls; the median age was 3.3 years (interquartile range 2.0-4.8); and 824 (51.1%) were first-born children, 578 (35.9%) were second-born children, and 209 (13.0%) were third-or-later born children. Coverage of non-NIP vaccines varied by sex and birth order, ranging from 58.2% for first-born boys to 28.4% for third-or-later born girls. Coverage among girls was less than among boys (PR: 0.91, 95% CI: 0.82-1.00), but the sex differences were not significant after adjusting for sociodemographic factors (PR: 0.94, 95% CI: 0.86-1.03). Compared with first-born children, coverage PRs were 0.85 (0.76-0.93) for second-born children and 0.82 (0.65-1.00) for third-or-later born children (p for trend < 0.001). Sex and birth order had joint effects on coverage, with the lowest coverage PRs in the third-or-later born girls (0.71, 95% CI: 0.49-0.97), compared to first-born boys. Multiplicative interactions of sex and birth order were significant (PR: 0.86, 95% CI: 0.75-0.98 in second-born girls).

Conclusions: Birth order of children was independently associated with non-NIP vaccines coverage, whereas sex showed no significant. However, sex and birth order were jointly associated with receipt of non-NIP vaccines, with later born children and girls having significantly lower coverage than first-born boys. In the context of the three-child policy in China, interventions to promote non-NIP vaccine equity should consider children's sex and birth order.