Mathematical Model of Susceptibility
The proportion of the population who are susceptible to a particular disease is denoted S. Due to the problems mentioned above, it is difficult to know this parameter for a given population. However, in a population with a rectangular population distribution (such as that of a developed country), it may be estimated by:
Where A is the average age at which the disease is contracted and L is the average life expectancy of the population. To understand the rationale behind this relation, think of A as the length of time spent in the susceptible group (assuming an individual is susceptible before contracting the disease and immune afterwards) and L as the total length of time spent in the population. It thus follows that the proportion of time spent as a susceptible is A/L and, in a population with a rectangular distribution, the proportion of an individual's life spent in one group is representative of the proportion of the population in that group.
The advantage of estimating S in this way is that both the average age of infection and average life expectancy will be well documented, and thus the other parameters needed to calculate S will be easily at hand.
The parameter S is important in the mathematical modelling of epidemics.
Read more about this topic: Susceptible Individual
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