Infectious diseases, including invasive bacterial infections, represent the most frequent cause of death among infants and young children worldwide.1–6  Immunization programs have been the most successful strategy to decrease the morbidity and mortality caused by infections in children across the world.7–9  However, despite the enormous success of the immunization programs, important knowledge gaps remain that need to be addressed to further optimize the global impact of childhood vaccinations. One aspect that has attracted considerable attention is the variability of individual immune responses.

In this issue of Pediatrics, Chapman et al.10  provide initial evidence of an important factor that may contribute to the differences in immune responses among young children after vaccination. Based on previous studies, mostly conducted in animal models, the authors hypothesized that the composition and diversity of the gut microbiome play a significant role in immune development and that by altering...

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