what-the-modellers-still-don’t-understand-about-herd-immunityWhat the Modellers Still Don’t Understand About Herd Immunity
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Should We Be Surprised That Case Numbers Have Been Falling?

Published On: 6. August 2021 9:50

“Scientists are scratching their heads over the precipitous decline in daily COVID-19 infections”, says a recent article in the journal Nature. “A sharp fall in the number of people testing positive has surprised scientists”, says a piece in the FT. According to the epidemiologist John Edmunds, “Nobody really knows what’s going on”.

Should scientists really be surprised by the fall in case numbers? Yes, some remaining restrictions were lifted on 19th July – the UK’s supposed ‘Freedom Day’. But cases have fallen in the absence of restrictions many times before. It’s therefore hardly surprising they would do so again.

To identify previous examples where infections fell in the absence of restrictions, I utilised the Oxford Blavatnik School’s COVID-19 Government Response Tracker. Specifically, I looked for examples where cases fell from a peak at a time when there were no mandatory business closures in place, and there was no mandatory stay-at-home order.

I was able to identify nine examples. (And note: one’s ability to identify examples is limited by the fact that almost all countries have had either mandatory business closures or a mandatory stay-at-home order in place during each successive wave of the virus.)

The nine examples are as follows: Sweden in the spring of 2020; Japan in the spring, the summer and the winter of 2020; North Dakota in the winter of 2020; South Dakota in the winter of 2020; Wyoming in the winter of 2020; Utah in the winter of 2020; and Iowa in the winter of 2020.

In all nine cases, infections fell in the complete absence of either mandatory business closures or a mandatory stay-at-home order. (Though in some of the cases, there were restrictions on large gatherings, or other less intrusive measures, in place.)

It should be noted that all these locations other than Japan have relatively low population densities – which presumably equates to lower transmission, all else being equal. (And Japan’s “success” in dealing with the virus may be due to some cultural or biological factor that is common to every country in South East Asia.) Nonetheless, differences in population density are of degree not of kind.

So what explains the declines – did people just change their behaviour voluntarily? Not necessarily, as I’ve noted before. In South Dakota, cases began falling rapidly in mid November, despite almost no government restrictions and little change in people’s overall mobility. How could this happen?

One possible explanation is super-spreaders. We know there is substantial variation in transmissibility across individuals. Most people don’t transmit the virus to anyone; but a few people spread it to many others. Perhaps cases start declining once enough of these super-spreaders have been infected.

Whatever the true explanation, lockdowns are not necessary for infections to start falling (even if they may cause this to happen slightly earlier or slightly faster than otherwise). Why, then, are the scientists so puzzled?

One reason, as Philippe Lemoine noted in our recent interview, is that some epidemiological models simply assume that only lockdowns can have a large effect on transmission. Not particularly scientific, you might say, but that’s modelling for you.

The fact that infections have been falling in the UK is actually even less surprising than I’ve suggested so far. That’s because over 93% of Britons now have COVID antibodies – acquired from either vaccines or natural infection (whereas in the examples listed above, the numbers were far lower).

In summary, a decline in case numbers is only surprising if you’re reasoning from a flawed model.


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what-the-modellers-still-don’t-understand-about-herd-immunityWhat the Modellers Still Don’t Understand About Herd Immunity
a-letter-from-icelandA Letter From Iceland