Background: It has been suggested that allergic sensitization is inversely related to the number of siblings in the family.
Objectives: To study whether a similar relation can be observed for hay fever and asthma among Finnish adolescents in a population with relatively low prevalence of atopic diseases.
Methods: A questionnaire mailed to a nationwide sample of 1849 families with 16-year-old twins assessing the cumulative incidence of doctor-diagnosed hay fever and asthma among the adolescents and the number of older siblings in the family by parental report.
Results: The cumulative incidence of hay fever was significantly lower among the adolescents with three or more older siblings (3.9%, 95% CI = 1.2-6.5%) compared with adolescents with fewer older siblings (12.7%, 95% CI = 11.4-14.0%). There was no difference in the cumulative incidence of asthma among the adolescents according to the number of older siblings in the family.
Conclusions: Large number of older siblings appears to be protective against the development of hay fever.