Thread by @WesPegden

This commentary on herd immunity thresholds is making the rounds today. There is nothing new in this article, and parts of its commentary are quite misleading. It is worth thinking about why a commentary like this is widely shared by scientists.
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1/11

nature.com/articles/s4157…

I worry that the ways in which the article is misleading is, in fact, precisely why it is widely shared.

E.g., they sow doubt for infection controlled immunity, suggesting that "several rounds of re-infection" might be needed to generate robust immune responses. 2/

This is not a reasonable level of uncertainty to apply in an article that also discusses vaccine-induced herd immunity as an inevitable eventuality.

Although they explain some of the mechanisms which can lower HIT's, they also say misleading things, e.g. that HITs are only... 3/

lowered if we expect that "it is always the same set of individuals that are potential super-spreaders." It is possible to stretch these statements into truths—in reality, heterogeneous contact models assume there is some durability in the *propensity* of people to interact,...

something which is very hard to question with a straight face—but phrasings are chosen here which, at the very least, are easily subject to misinterpretations by anyone who might have something to learn from this article.

Most problematic is that the authors suggest there's..5/

"little evidence" HITs for COVID-19 might be below 50%. It's particularly inappropriate to suggest this while failing to mention the only high-profile work I am aware of which has attempted to actually make any HIT predictions for particular countries.6/

We have a situation where the only people who have tried to predict HITs from the data have concluded that they are much less than 50% for several European countries, and then other people ignore this work, offer no competing predictions, and say we lack evidence.

Beyond...
7/11

... the question of what HITs we should expect for COVID-19, the article also adopts a very misleading approach to analyzing the mortality which would result from immunity levels significant enough to control the epidemic. They ignore the significance of age in both... 8/11

transmission patterns and mortality, use a crude IFR range of .3%-1.3%, and suggest that herd immunity would inevitably (in their most optimistic scenarios, where we don't usually need multiple rounds of infection for robust immunity!) result 500k-2.1 million deaths.

Note...9/11

that it would seem reasonable for a reader to conclude that these authors believe that having 500k or fewer fatalities from HI is as unlikely as having 2.1 million or more.

Producing even half-reasonable estimates like this for COVID-19 requires, at a minimum, a basic 10/11

age-stratified epidemic model. Alarmist over-estimates are not an acceptable substitute.

We should not tolerate distortions of science, even when they could be used to support our favorite policy positions.
11/11

Addendum:

Let me mention just one more anti-scientific aspect of this piece, which has featured prominently in many attempts to dismiss suggestions that HIT's can be low.

The authors of this article point out that there is a boat, where 70% of people became infected. 12/

In particular, they bring this up to attack what can be fairly characterized as a "strawman"; namely that 50% of humans are perfectly immune to infection by COVID-19, in the sense that they would never test PCR+ in any situation.

But the huge impact of heterogeneity on HITs..13/

does not depend this kind of "perfectly immune" population.

Biological variation in susceptibility affects HITs to the extent that the *probability* an individual is infected at a given exposure level varies at all from individual to individual. As with contact patterns, ...14/

heterogeneity in susceptibility should be our expectation; perfectly equal infection susceptibility would be a remarkable feature of the human species which would require some explanation.

Even if the strawman has no relevance, does the boat with 70%+ still tell us something?15/

Any epidemiologist who studies herd immunity thresholds would tell you that we should not expect HITs or final epidemic sizes to be the same in hyperlocal environments like boats, meatpacking plants, or nursing homes and towns, cities, and whole countries. Indeed, ... 16/

the HIT of COVID is a property not just of the infectious agent itself, but of our society and its interaction patterns; indeed, this is why it we should also not expect HITs to agree in all places and be careful about comparing, say, Manhattan with the country of Portugal 17/

(let alone comparing a country to a boat). 18/18

I agree with this suggestion; I certainly do not mean to "subtweet" anyone and would welcome a discussion with @SCauchemez and/or Fontanet about this article.

The same goes for those retweeting it, e.g. @michaelmina_lab, @AdamJKucharski, etc.



19/18