title: The causal direction in the association between respiratory syncytial virus hospitalization and asthma.
abstract: BACKGROUND
  Earlier studies have reported an increased risk of asthma after respiratory syncytial virus (RSV) hospitalization. Other studies found that asthmatic disposition and propensity to wheeze increase the risk of RSV hospitalization.
  
  
  OBJECTIVE
  The current study examined the causal direction of the associations between RSV hospitalization and asthma in a population-based cohort of twins.
  
  
  METHODS
  We conducted a prospective cohort study examining the associations between RSV hospitalization and asthma by using registry information on RSV hospitalization and asthma among 18,614 Danish twins born 1994 to 2003. The associations between RSV and asthma were examined in both directions: we examined the risk of asthma after RSV hospitalization, and the risk of RSV hospitalization in children with asthma in the same population-based cohort.
  
  
  RESULTS
  Asthma hospitalization after RSV hospitalization was increased as much as 6-fold to 8-fold during the first 2 months after RSV hospitalization but was no longer increased 1 year later. Asthma increased the risk of RSV hospitalization by 3-fold, and the risk was not time-dependent. Analyzing these associations on the basis of asthma defined from use of inhaled corticosteroid did not materially change the risk estimates.
  
  
  CONCLUSIONS
  There is a bidirectional association between severe RSV infection and asthma. Severe RSV infection is associated with a short-term increase in the risk of subsequent asthma, suggesting that RSV induce bronchial hyperresponsiveness; and asthma is associated with a long-term increased susceptibility for severe RSV disease, suggesting a host factor being responsible for the severe response to RSV infection. This suggests that severe RSV infection and asthma may share a common genetic predisposition and/or environmental exposure.
authors.affiliation: Bandim Health Project, Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark. LGN@ssi.dk
authors.forename: Lone Graff
authors.initials: LG
authors.lastname: Stensballe
authors.forename: Jacob Brunbjerg
authors.initials: JB
authors.lastname: Simonsen
authors.forename: Simon Francis
authors.initials: SF
authors.lastname: Thomsen
authors.forename: Anne-Marie Hellesøe
authors.initials: AM
authors.lastname: Larsen
authors.forename: Susan Hovmand
authors.initials: SH
authors.lastname: Lysdal
authors.forename: Peter
authors.initials: P
authors.lastname: Aaby
authors.forename: Kirsten Ohm
authors.initials: KO
authors.lastname: Kyvik
authors.forename: Axel
authors.initials: A
authors.lastname: Skytthe
authors.forename: Vibeke
authors.initials: V
authors.lastname: Backer
authors.forename: Hans
authors.initials: H
authors.lastname: Bisgaard
mesh_terms: D001249:Asthma / Q000453:epidemiology* / Q000821:virology; D002648:Child; D002675:Child, Preschool; D003718:Denmark; D004781:Environmental Exposure; D005260:Female; D020022:Genetic Predisposition to Disease; D006760:Hospitalization; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D012042:Registries; D018357:Respiratory Syncytial Virus Infections / Q000453:epidemiology*; D012136:Respiratory Syncytial Viruses; D012307:Risk Factors; D013997:Time Factors