Objective. To study the occurrence of bronchial obstructive symptoms and immunoglobulin (Ig) E antibodies after respiratory syncytial virus (RSV) bronchiolitis in infancy. Previous studies of this subject have mostly been retrospective or without controls, or the controls have not been followed prospectively.

Design. This was a prospective cohort study with matched controls.

Participants. Forty-seven infants had experienced RSV bronchiolitis severe enough to cause hospitalization at a mean age of 3½ months. For each child with RSV infection, two controls were acquired from the local Child Health Center and matched for date of birth, sex, and residence. Only one control was obtained for one RSV child, and the control group thus contained 93 children.

Methods. All the children underwent two follow-up examinations, the first one at a mean age of 1 year and the second at a mean age of 3 years. At the first follow-up, a skin-prick test against egg white was performed, and serum IgG antibodies against RSV were measured. At the second follow-up, serum IgE antibodies were measured using screening tests for common food and inhalant antibodies, and skin-prick tests against egg white, cat, birch, and mite allergen were performed. Hereditary and environmental factors (passive smoking, indoor furred animals) and duration of breast-feeding were recorded.

Results. At the first follow-up, 89% in the RSV group and 27% in the control group had IgG antibodies against RSV (P < .001). At the second follow-up, asthma, defined as three episodes of bronchial obstruction verified by a physician, was found in 11 of 47 children (23%) in the RSV group and in 1 of 93 children (1%) in the control group (P < .001). A positive test for IgE antibodies was noted in 14 of 44 (32%) RSV children and in 8 of 92 (9%) children in the control group (P = .002). An analysis of risk factors for the development of asthma and IgE antibodies on the whole group of 140 children showed that RSV bronchiolitis was the most important risk factor, and a family history of atopy or asthma further increased the risk.

Conclusions. Respiratory syncytial virus bronchiolitis during the first year of life apparently is an important risk factor for the development of asthma and sensitization to common allergens during the subsequent 2 years, particularly in children with heredity for atopy/asthma.

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