HAY-FEVER, ASTHMA AND NUMBER OF OLDER SIBLINGS - A TWIN STUDY

Background It has been suggested that allergic sensitization is invers ely related to the number of siblings in the family. Objectives To stu dy whether a similar relation can be observed for hay fever and asthma among Finnish adolescents in a population with relatively low prevale nce of atopic diseases. Methods A questionnaire mailed to a nationwide sample of 1849 families with 16-year-old twins assessing the cumulati ve incidence of doctor-diagnosed hay fever and asthma among the adoles cents and the number of older siblings in the family by parental repor t. Results The cumulative incidence of hay fever was significantly low er 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 incid ence of asthma among the adolescents according to the number of older siblings in the family. Conclusions Large number of older siblings app ears to be protective against the development of hay fever.