Causal direction between respiratory syncytial virus bronchiolitis and asthma studied in monozygotic twins.

Background: Respiratory syncytial virus (RSV) bronchiolitis has been associated with later development of asthma, wheezing, abnormal pulmonary function, and sensitization. Our aim was to determine the differential effect within monozygotic (MZ) twin pairs discordant for severe RSV bronchiolitis in infancy on the subsequent development of asthma, pulmonary function, and allergy.

Methods: Thirty-seven MZ twin pairs discordant for RSV hospitalization in infancy (mean age 10.