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The polio model. Does it apply to polio?
Nete Munk Nielsen1, Peter Aaby, Jan Wohlfahrt
1Department of Epidemiology Research, Danish Epidemiology Science Centre, Statens Serum Institut, Copenhagen, Denmark. NMN@SSI.dk
Insights
The polio model
Area of Science:
- Epidemiology
- Infectious Disease Modeling
- Public Health
Background:
- The polio model suggests disease severity increases with age at infection.
- This implies firstborns and those in small families face higher risks due to later infection.
- Contradictory explanations, like the intensive-exposure model, exist for childhood infection severity.
Purpose of the Study:
- To evaluate the polio model's predictions regarding age, birth order, and family size.
- To compare the polio model with the intensive-exposure model for polio severity.
- To analyze the epidemiology of polio infection and its risk factors.
Main Methods:
- A retrospective study of 5590 historical polio cases (1940-1953) in Copenhagen.
- Evaluation of polio incidence and severity (paralysis, mortality) based on age, birth order, and sibship size.
- Utilized census data from 1940 and 1950 for relative risk calculations.
Main Results:
- Polio severity showed a U-shaped curve, highest in the youngest and oldest age groups, not a steady increase.
- Higher polio incidence observed in larger families (RR=1.13) and for later-born children (P<0.0001).
- Second-born children (1-4 years) had higher paralytic polio risk (RR=1.47), while older children (>=5 years) had lower risk (RR=0.65), supporting intensive exposure.
Conclusions:
- The polio model's predictions on age, sibship size, and birth order received limited support.
- An intensive-exposure model may better explain polio infection epidemiology and severity.
- Findings suggest complex interactions between age, family structure, and polio risk.
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).
Conclusion:
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.