title: Infectious burden of respiratory syncytial virus in relation to time of birth modifies the risk of lower respiratory tract infection in infancy: the Norwegian Mother and Child Cohort.
abstract: OBJECTIVE
  The risk of respiratory syncytial virus (RSV) infection according to calendar month of birth has previously been demonstrated. We hypothesize that the real-time infectious burden (IB) of RSV, in relation to time and county of birth, modifies the risk of lower respiratory tract infection (LRTI) in infants, an association that has not been investigated previously, and may be used as an alternative measure of RSV risk when drafting RSV immunoprophylaxis guidelines.
  
  
  METHODS
  Thirty thousand eighty-seven pregnancies were included from July 1, 2003, to July 31, 2006. IB was defined by month and county as the number of RSV detections per inhabitant. IB for 5 periods ante- and postnatally was calculated for each child. Outcome variables were maternally reported hospitalization for LRTI before age 6 months and LRTI before age 12 months. Logistic regression was used to estimate associations between IB and LRTI.
  
  
  RESULTS
  The odds of hospitalization for LRTI rise with increased IB the first 3 months after birth. Low IB: odds ratio (OR) 1.17 (95% confidence interval [CI]: 0.98-1.39); medium IB: OR 1.42 (95% CI: 1.21-1.68); high IB: OR 2.51 (95% CI: 2.15-2.94). High IB 3-0 months before birth confers a lower odds of hospitalization for LRTI the first 6 months of life, OR 0.51 (95% CI: 0.43-0.61). Similar results were seen for maternally reported LRTI the first 12 months of life.
  
  
  CONCLUSIONS
  We find an association between real-time RSV infectious burden and LRTI in infancy: high burden before birth is protective and high burden after birth increases the risk.
authors: Department of Pediatric and Adolescent Medicine, Akershus University Hospital, Lørenskog, Norway.
authors: Ida Monn
authors: IM
authors: Birkhaug
authors: Christopher Stephen
authors: CS
authors: Inchley
authors: Geir
authors: G
authors: Aamodt
authors: Gabriel
authors: G
authors: Ånestad
authors: Wenche
authors: W
authors: Nystad
authors: Britt
authors: B
authors: Nakstad
mesh_terms: D015331:Cohort Studies; D005260:Female; D005843:Geography; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D009664:Norway / Q000453:epidemiology; D011247:Pregnancy; D012141:Respiratory Tract Infections / Q000453:epidemiology*; D018570:Risk Assessment; D013997:Time Factors