All individuals in the EMOD typhoid model start out in the susceptible state. However, some individuals may have immunity to typhoid, which will prevent them from becoming infected. The model includes two types of immunity: age-specific and acquired.
For age-specific immunity, individuals are fully immune at birth and move from immune to susceptible in three age bins: at 6-months of age, 3 years of age, and 6 years of age. No children under the age of 6 months can become infected. The model allows for specification of the susceptible fraction for each of the age bins, using the configuration parameters Typhoid_<age bin>_Susceptible_Fraction. At each of the ages specified in the bins, a proportion of the remaining immune population will be moved to the susceptible population.
Acquired immunity is implemented as a multiplier in the probability of infection for both clinical and subclinical individuals. In the model, a counter starts at zero at birth and tracks the total number of infections during a lifetime. The probability of becoming susceptible is reduced following each infection, and is configurable with the configuration parameter Typhoid_Protection_Per_Infection.