FAMILY-STRUCTURE, NEONATAL INFECTION, AND HAY-FEVER IN ADOLESCENCE

Objective-To determine whether increased numbers of siblings and infec tion in early life protect against allergic sensitisation. Design-Hist orical cohort study. Setting-Sheffield, UK. Subjects-11 765 children a ged 11-16 years for whom a history of neonatal infectious illness had been recorded systematically at 1 month of age. Methods-A history of h ay fever and family structure was obtained by postal questionnaire; ne onatal illness history was ascertained from health visitor records; 72 3 children underwent skin prick testing with mixed grass pollen extrac t.Results-The prevalence of hay fever was reduced (p<0.0001) among chi ldren of younger mothers, and those from larger families. The number o f older siblings exerted a stronger independent effect than the number of younger siblings (p<0.001). Infants breast fed exclusively during the first month were at higher risk (p<0.05) of subsequent hay fever, independent of demographic factors. Adolescents at high risk of hay fe ver by virtue of their family structure were more likely to be sensiti sed to grass pollen (p<0.002). No significant relations emerged betwee n hay fever and infection in the first month of life, even among child ren born in June. Conclusions-The association of hay fever with family structure is not due to reporting bias and reflects an environmental influence on allergic sensitisation. The effects of sibship size, birt h order, and infant feeding are consistent with a protective influence of postnatal infection. The first month of life and the first postnat al exposure to allergen are not the critical periods during which this protective effect is determined.