"Just as the departure of workers from cities raised alarm bells over covid, so too does their return. But this time, the concern is not that these individuals may bring back infection: prevalence is still higher in urban areas than in rural areas. Rather, the concern is that their return will bring back more people to get infected.
The spread of the infection depends not just on the number of infected, but the fraction who are susceptible. The standard susceptible-infected-recovered (SIR) model that is the workhorse of quantitative epidemiology, shows that the number of infected depends on: one, the probability that contact between an infected and susceptible person leads to infection; two, the number of infected people; and three, the fraction of people who are susceptible. This last factor is the reason that susceptible persons are sometimes called the “fuel" for epidemics."
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