Birth order and infant health: Evidence from maternal immunisation in New Zealand.

Immunisation during pregnancy is a vital strategy to protect infants from infectious diseases in their first months of life, yet little is known about how birth order relates to maternal vaccination uptake and infants' risk of severe illness. Drawing on administrative data from New Zealand covering more than 200,000 births between 2015 and 2023, I analyse the relationship between birth order and maternal vaccination against pertussis and influenza. I also examine subsequent infant hospitalisations for these diseases. The findings show that later-born children experience higher hospitalisation rates, likely because of increased exposure to infectious diseases through older siblings. At the same time, maternal vaccination rates decline with each pregnancy, leaving those who would benefit most from maternal immunisation the least likely to receive it. These findings suggest that improving vaccination uptake during later pregnancies could yield important health gains for infants at highest risk.