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Cohort study of sibling effect, infectious diseases, and risk of atopic dermatitis during first 18 months of life

BMJ 2004; 328 doi: (Published 20 May 2004) Cite this as: BMJ 2004;328:1223
  1. Christine Stabell Benn, research fellow (cb{at},
  2. Mads Melbye, professor of epidemiology1,
  3. Jan Wohlfahrt, statistician1,
  4. Bengt Björkstén, professor of paediatrics and allergy prevention3,
  5. Peter Aaby, professor of international health2
  1. 1 Department of Epidemiology Research, Danish Epidemiology Science Centre, Statens Serum Institut, Denmark,
  2. 2 Projecto de Saúde de Bandim, Bissau, Guinea-Bissau
  3. 3 Center for Allergy Research and Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden
  1. Correspondence to: C Benn
  • Accepted 5 March 2004


Objectives To determine whether early infectious diseases could explain the association between number of siblings and other markers of microbial exposure and the development of atopic dermatitis before the age of 18 months.

Design Cohort study. Information on atopic dermatitis, infectious diseases occurring before 6 months of age, number of siblings, early day care, pet keeping, farm residence, and background factors was collected in telephone interviews.

Setting Danish national birth cohort.

Participants 24 341 mother-child pairs.

Main outcome measures Incidence rate ratios of atopic dermatitis.

Results 13 070 children (54%) had at least one clinically apparent infectious disease before 6 months of age. At age 18 months, 2638 (10.8%) of the children had had atopic dermatitis. The risk of atopic dermatitis increased with each infectious disease before 6 months of age (incidence rate ratio 1.08, 95% confidence interval 1.04 to 1.13). The risk of atopic dermatitis decreased with each additional exposure to three or more siblings, day care, pet ownership, and farm residence (0.86, 0.81 to 0.93).

Conclusions Early infections do not seem to protect against allergic diseases. The protective effect of number of siblings, day care, pet ownership, and farm residence remained after adjustment for clinically apparent infectious diseases, suggesting that the effect is established independently early in life.


  • Contributors CSB, MM, BB, and PA were involved in drafting the study protocol. CSB and JW analysed the data. All authors were involved in interpreting results and writing the paper. CSB, MM, and PA are the guarantors. Funding: Danish National Research Foundation, Pharmacy Foundation of 1991, Egmont Foundation, March of Dimes Birth Defects Foundation, Augustinus Foundation, Leo Foundation, and Aage Bang's Foundation.

  • Competing interests None declared.

  • Ethical approval Ethical committees in Denmark and by the Data Protection Board. The steering committee for the Danish National Research Foundation approved the use of data for the present study.

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