A protective gap is widening precisely where it matters most: the infants at greatest biological risk of severe infectious illness are the least likely to be shielded by maternal vaccination. This counterintuitive pattern — where medical need and preventive care move in opposite directions — has real implications for pediatric health outcomes across developed nations with similar immunization structures.
Drawing on New Zealand administrative records covering more than 200,000 births from 2015 to 2023, this study examined how birth order shapes both maternal vaccination uptake against pertussis and influenza and subsequent infant hospitalizations for those diseases. Later-born children showed measurably higher hospitalization rates for both conditions, consistent with the well-documented "sibling transmission" effect: older children circulating in schools and social environments bring pathogens home to vulnerable newborns. Critically, maternal vaccination rates fell with each successive pregnancy — meaning the infants with the greatest household exposure risk were least protected by transplacental antibody transfer, the primary immunological defense available before infants can receive their own vaccines.
This inverse relationship between risk and protection is not a trivial finding. Maternal immunization against pertussis and influenza has robust evidence behind it; transplacentally transferred antibodies confer meaningful protection in the first months of life when infant immune systems are most immature. What this study adds is a structural equity dimension: the healthcare engagement patterns that typically accompany later pregnancies — fewer prenatal visits, less novelty-driven vigilance — appear to translate into a measurable vaccination gap. The study is observational and limited to one country, so causal inference is constrained. However, the large population-level dataset and the consistency of the birth-order gradient across both diseases lend the findings credibility. For health systems designing targeted outreach, these results suggest that multiparity itself may serve as a useful proxy for identifying under-vaccinated pregnancies.