Database Reference
In-Depth Information
1: TOTAL POPULATION
2: NUMBER SUSCEPTIB…
3: NUMBER INFECTED
4: NUMBER RECOVERED
1:
100
1
1
1
1
2:
3:
4:
4
4
4
4
1:
2:
50
3:
4:
2
3
2
1:
2
2
2:
3:
3
3
3
4:
0
0.00
25.00
50.00
75.00
100.00
Days
Fig. 12.3
of susceptible individuals is low enough that contacts between infected individuals
and the remaining susceptible individuals are rare enough that transmission declines
to zero. If we run our simple dynamic SIR model with both the BIRTH RATE and
DEATH RATE set to zero, we get the classical population-static form of the model,
which produces a classical epidemiological curve for a limited disease outbreak
(Figure 12.3). Note how the TOTAL POPULATION (light gray) stays constant and
the outbreak ends when the majority of the population is immune to the disease (i.e.
majority of individuals are RECOVERED).
However, many infectious diseases occur over time periods in which the addition
of new susceptible individuals through births might drastically alter the course of the
disease. What happens to the epidemic curve when the population is no longer static,
but growing steadily? (Figure 12.4). With the constant addition of new susceptible
individuals, the disease persists in the population instead of going “extinct,” as in
the previous run of the model. Note how population size (light grey) is increasing
with a slow exponential curve. In the population-static model the disease disappears
around day 50. In the population-dynamic SIR model, however, a disease can cycle
or become a persistent element where the model reaches a stable equilibrium in
which the disease persists at low prevalence, depending on the rate of population
growth.
12.2.2 Questions and Tasks
As we saw in this section, population dynamics can alter the course of the disease,
resulting in persistence in the population.
