Germ factories or immune boot camps? Infection and immunity in childcare settings

SUMMARYChildcare outside the home is a common experience for many children in high-income countries. It is associated with an increase in the incidence of infectious diseases-not just for the child but also for their parents and other household members. In this review, we explore this phenomenon from multiple angles, combining age at first infection, maternal antibody dynamics, seroepidemiology, cohort studies, and outbreak reports to understand the relationship between the immune systems of children starting childcare and the pathogen milieu they encounter. We consider the interaction between the age at which many infants begin out-of-home childcare and the maturation of cellular and humoral immunity. We bring together data on what is "normal" for infections in the first years of life: the range and incidence of gastrointestinal, respiratory, and rash-forming illnesses that typically infect young children. We review evidence of the additional impact that childcare has on the transmission of these pathogens. The economic and personal impacts of these illnesses are considered, including our lived experiences. We ask whether there are effective interventions to reduce illness associated with childcare, as the UK adds chickenpox to the childhood vaccination schedule. Finally, we consider evidence suggesting a trade-off between infections earlier in life and when children start formal schooling. We conclude that the high burden of infection in young children is normal, linked to the immunobiology of infants and the transmission dynamics of individual pathogens, but the collective impact of these infections is underappreciated.