 ## SIR model with 2 age classes (page 79) It is now possible to extend the formulation developed for the risk-structured SIS models to an age-structured SIR model.  This involves three major changes:
1) Given that we are now dealling with an SIR model, (at least) two compartments need to be modelled per age group.  (In fact we should model S, I and R in each age-group and then use the fact that the sum of all the population compartments equals 1 to eliminate one of the variables. However, by assuming that the size each of the age-classes (nC and nA) is at equilbrium, we can simplify the dynamics and just consider two variable per age-group.)
2) Demography needs to be included, this essentially means that individuals need to be born into the youngest age-group. In addtion, natural mortality can be age-structured.
3) Finally, individuals move through the age-classes as they mature. In the two age-class example below individuals leave the childhood class and move into the adult class, taking their infectious state with them.

In this set of programs we consider the simplest case where just two age-groups (children and adults) are models. Again we use a frequentist, such that SC, IC and RC refer to the proportion of the entire population that are susceptible,  infectious or recovered and also in the childhood age-class. Putting these elements together, we arrive at the following differential equations: Parameters
 β is the matrix of transmission rates and incorporates the encounter rate between susceptible and infectious individuals together with the probability of transmission. γ is called the removal or recovery rate, though often we are more interested in its reciprocal (1/γ) which determines the average infectious period. lC is the rate at which children mature and move into the adult age class nC is the proportion of the population that are in the childhood group. This is not defined explicitly but calculated as:  nC = μA/(lC+μA) μC is the death rate in the childhood group μA is the death rate in the adult group ν is the birth rate into the childhood class. This is not defined explicitly but calculated as:  ν = (lC+μA) nC SC(0) is the initial proportion of the population that are both susceptible and in the childhood group. IC(0) is the initial proportion of the population that are both infectious and in the childhood group. SA(0) is the initial proportion of the population that are both susceptible and in the adult group IA(0) is the initial proportion of the population that are both infectious and in the adult group.
All rates are specified in years.

Requirements.
All parameters must be positive, SC(0) + IC(0)≤ nC, and SA(0) + IA(0)≤ nA = 1- nC

Files
C++ ProgramPython ProgramFortran ProgramParametersMATLAB Code.

 Questions and comments to: M.J.Keeling@warwick.ac.uk or rohani@uga.edu Princeton University Press Our research web pages: Matt Keeling      Pejman Rohani