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.
A leaked CDC document has been circulating today with a claim that the Delta variant is as contagious as chickenpox. That news is enough to make anyone worried. The fact that this variant is also more likely to result in acute sickness is doubly worrying. With 110,000 confirmed breakthrough cases in the USA there is a palpable sense of worry that news is about to get worse. Meanwhile, the UK’s number of cases has peaked and is falling quickly. That should act as at least a partial salve to those worries.Click HERE to subscribe to Fuller Treacy Money Back to top