This lack of long-term protection against infection means that herd immunity is probably impossible and that the virus will become endemic and continue to circulate in human populations. If this happens and the disease then stabilises, such that case numbers are constant across the population, neither increasing nor decreasing, it will have reached what’s called an “endemic equilibrium”.
So is this what we’re now witnessing? Possibly. One of the basic models of how infectious disease cases change over time is called an SIR model, which looks at how many people are susceptible to a disease, infectious with it or have recovered from it (and so are immune) at any one time.
With this model, cases increase rapidly at the start of an epidemic as lots of people are susceptible, become infected, and go on to infect other susceptible people. But as infections mount, over time fewer people are susceptible and more have recovered. The rate of growth therefore decelerates, the epidemic reaches its peak, and then case numbers decline to an endemic equilibrium point, where they remain roughly stable.
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