The first thing to emphasise is that this fall is not the result of some special temporary factor. It isn’t that there was a week of glorious sunshine, the temporary introduction of a new set of restrictions, a sudden change in the testing rules or even the dropping out of the numbers of a previous shock that had temporarily raised numbers. It’s simply that the collective immunity we now have, through a combination of vaccines and people recovered from illness, is sufficient that, given the way we behave (eg more working from home) and given the time of year (August) the virus cannot find enough susceptible people to infect for its rate of spread to accelerate. That means, that, by definition, we have reached what is called the “herd immunity threshold” (HIT) – the collective percentage immunity, across the whole population, at which the virus can no longer sustainably spread.
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