The version of the SIR model discussed in Section 21. 2 is flawed because it does not account for the possibility of prevalence elasticity.

In Section 21.2 The SIR model of infectious disease

Indicate whether each statement is true or false, and justify your answer.

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In this section, we introduce a basic epidemiological model of infectious disease, known as the SIR model (Anderson and May 1985). This model traces how an infectious disease evolves over time. Such models help us to understand when a population is at its most vulnerable, when an epidemic is at its most virulent, and when its effects are likely to wane. They also help measure the effect of vaccinations and public health interventions such as vaccine subsidies. The SIR model provides a convenient framework for thinking about the reciprocal relationship between prevalence and self-protection. In the SIR model, the members of a population are classified into three groups: sus- ceptible, infected, and recovered, with sizes St, It, and R, at time t. As time passes, babies are born, people die, the healthy become sick, and the sick recover. S₁, I, and R, fluctuate accordingly. To study the model's dynamics, we first need to enumerate all the ways that members of the population can move from group to group. Members of the susceptible population S fall ill and join the infected group I at the infection rate λs. Analogously, the sick recover and move from the infected group I to the recovered group R at the recovery rate. In this version of the model, A, babies are born each period, and members from all three groups die at their respective rates Ads, and Ad. Lastly, the vaccination rate v reflects movement from the susceptible group S to the recovered group R. Figure 21.3 displays each of these pathways and their corresponding rates.