title: The polio model. Does it apply to polio?
abstract: BACKGROUND
  According to the polio model, severity of disease increases with age at infection. Firstborn children and people belonging to small families are generally infected later and should accordingly have a higher risk of severe polio. However, this model does contradict other explanations of severity of childhood infections including the intensive-exposure model.
  
  
  METHODS
  To evaluate the deductions from the polio model we performed a study based on medical records from 5590 historical polio cases from the county of Copenhagen 1940-1953. The relative risk (RR) of polio according to age, birth order and sibship size was evaluated using census data from 1940 and 1950.
  
  
  RESULTS
  Severity of polio measured as frequency of paralysis or mortality did not show a steady increase with age, but a U-shaped curve being highest for the youngest as well as the oldest patients. The incidence of polio and paralytic polio was higher in families with several children compared with single children (RR = 1.13, 95% CI : 1.0-1.3). Furthermore, the incidence was higher in later-born children (P(trend) < 0.0001). However, as predicted from the intensive-exposure model, second-born children aged 1-4 years in two-child families had a higher risk of paralytic polio than first-born children (RR = 1.47, 95% CI : 1.1-2.0), whereas the opposite relationship was found for those aged > or = 5 years (RR = 0.65, 95% CI : 0.5-0.9).
  
  
  CONCLUSIONS
  The polio model's prediction about the impact of age, sibship size and birth order on polio incidence and severity found only limited support. A model emphasizing intensity of exposure as a risk factor for severity may account better for the epidemiology of polio infection.
authors.affiliation: Department of Epidemiology Research, Danish Epidemiology Science Centre, Statens Serum Institut, Copenhagen, Denmark. NMN@SSI.dk
authors.forename: Nete Munk
authors.initials: NM
authors.lastname: Nielsen
authors.forename: Peter
authors.initials: P
authors.lastname: Aaby
authors.forename: Jan
authors.initials: J
authors.lastname: Wohlfahrt
authors.forename: Kåre
authors.initials: K
authors.lastname: Mølbak
authors.forename: Mads
authors.initials: M
authors.lastname: Melbye
mesh_terms: D000293:Adolescent; D001722:Birth Order; D002648:Child; D002675:Child, Preschool; D003718:Denmark; D005191:Family Characteristics; D006801:Humans; D007223:Infant; D008962:Models, Theoretical; D011051:Poliomyelitis / Q000453:epidemiology*; D012189:Retrospective Studies; D012307:Risk Factors; D012720:Severity of Illness Index; D012923:Social Class