Biology Reference
In-Depth Information
3.5.2 biological Modeling
The most classical approach for modeling epidemiological phenomena is
biology-based modeling. Formulation is typically derived from the princi-
ples of disease transmission and based on the current knowledge of biologi-
cal mechanisms. However, it is more often the case that not much disease
biology is known and replacements with assumptions are necessary. As
such, an advantage of this method is that one can test the current under-
standing of disease transmission using the model; a discrepancy between
the model outcome and data would indicate a missing process. In the fol-
lowing, we will discuss three biological-based modeling methods: the com-
partmental SEIR model, agent-based simulation, and Markov chain Monte
Carlo (MCMC).
3.5.2.1 SEIR
One of the most widely used biological modeling frameworks is the compart-
mental, deterministic, ordinary differential equation (ODE)-based model
describing the dynamics of the host population. Hosts are divided into
categories such as susceptible, exposed, infected, and recovered (SEIR), as
represented by the state variables. The “exposed” compartment is for those
who are in the latent period, when one is infected but not yet infectious;
“recovered” is for those who have gained immunity. Immunity can either
be permanent or temporary, depending on the disease control. In the case of
temporary immunity, the recovered individual may go back into the “sus-
ceptible” class. There are various ways to divide the host population, and
this section only describes the method that is most commonly used along
with the general formulation. Readers, who are interested in the detailed
treatment of the SEIR model, are referred to Anderson and May (1991).
For a directly transmitted disease (Figure 3.1), any susceptible individual
moves to the exposed class when contact with the infectious class results in
disease transmission. This transmission is conventionally formulated using
Susceptible
(S)
Exposed
(E)
Infectious
(I)
Recovered
(R)
Birth
Death
Death
Death
Death
Figure 3.1
Diagram representing a simple SEIR model for a directly transmitted disease in a closed popu-
lation. The model assumes the disease is not inheritable, and therefore, all births go into sus-
ceptible class. Transition from susceptible to exposed is due to contact with the infectious, as
indicated by the dashed line.
