# Babies With Older Siblings Face Higher Infection Risk, Lower Vaccination Protection

Maternal antibodies decline with each pregnancy, leaving later-born children more vulnerable to infectious disease despite vaccination efforts, according to new research.

The study tracks a well-documented biological pattern: mothers pass fewer protective antibodies to their second, third, and subsequent children compared to firstborns. This phenomenon occurs because maternal antibody levels naturally decrease over time, particularly when pregnancies happen close together.

The findings carry real consequences. Younger siblings enter the world with weaker passive immunity from their mothers, a protective shield that typically lasts several months before vaccines take full effect. Simultaneously, these children face higher exposure to infections through older siblings who attend school and daycare. The combination creates a vulnerable window.

Vaccination rates matter in closing this gap. When mothers receive vaccines during pregnancy—such as pertussis, flu, and respiratory syncytial virus (RSV) shots—they boost antibody transfer to newborns. Yet maternal vaccination rates decline with parity, meaning fewer mothers of later-born children receive these protective shots. This compounds the biological disadvantage.

The research emphasizes an often-overlooked public health message: maternal vaccination protects not just the pregnant person, but the newborn during their earliest, most fragile months. Public health campaigns have historically centered on routine infant vaccines starting at birth, but the research suggests earlier protection through maternal vaccination deserves greater attention.

Healthcare providers caring for families with multiple children should prioritize vaccination conversations with pregnant patients, particularly those expecting younger siblings. Earlier-born children also benefit from recommended vaccines on schedule, reducing household transmission risk to newborns.

The findings underscore why vaccination rates among pregnant people remain a policy and education priority. Public health agencies continue promoting maternal vaccines despite ongoing vaccine hesitancy in some communities. For families planning multiple pregnancies close together, spacing pregnancies further apart and