Abstract Background Incidence of allergies among children is increasing and is an important public health issue. We examined whether early life immune modulating exposures and surrogate measures are associated with a reduced risk of allergic diseases among Japanese children. Methods We analyzed data from the Health Diary Study comprising a nationally representative cross-sectional sample of children/adolescents aged 1 to 17 years in Japan. Data were collected on early life factors and diagnosed allergic diseases using a self-administered questionnaire. Multivariable logistic regression was conducted evaluating the effects of delivery method, breastfeeding, infections in infancy, older siblings, and daycare on the risk of any allergy and specific types. Results Among 744 children included (mean age=9.5 years; 52% males), 41% experienced an allergic condition. Infection in infancy was associated with an increased risk of asthma (OR = 1.77, 95% CI: 1.09-2.85) especially related to bronchitis and ear infection, and showed a dose-response effect with increasing number of infections (p=0.04). Having older sibling(s) modified the effect of infections on asthma risk (p-interaction=0.11). Reduction in risk of allergic rhinitis was suggestive related to exclusive breastfeeding and having two or more siblings. Conclusions Evidence is unclear regarding a protective role of early life immune modulatory exposures in allergy risk in Japanese children. In contrast, infectious episodes in infancy may increase the risk of asthma. Key messages While multiple mechanisms may be at play, infections in infancy may increase the risk of asthma which supports protection against infectious transmissions.
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