The fragility of herd immunity

Trouble in the Madrid region is brewing again, even though earlier seroprevalance had clocked in at about 20 percent:

Good for New York of course, here is a thread discussing the comparison, to me the conclusions seem premature.  The important point in any case is that Covid-protected time periods need not last forever, and you can end up in multiple rounds of “let it rip.”  As far as I know, this is the first established case of a major “second wave” in a previously hard-hit area.

The good news is that Madrid cases seem to have peaked, and furthermore the death rate is much lower the second time around, the latter being one good reason for postponing cases into later time periods rather than taking them all up front.

Note also that England has had months of open pubs, and a very quiet situation, but now cases there are doubling every six to seven days (FT).  Don’t switch back to talk of deaths!  The “simple” theory of herd immunity is surprised to see that new trend in cases.  What I call semi-herd immunity suggests a high degree of protection for the current configuration of social relations, after some point.  As those social relations change, some of that temporary herd immunity dissolves, as new infecting connections are being created and new superspreaders arise and do their thing.  But that takes a while, possibly months.

The herd immunity theorists downplay the possible temporariness of the equilibrium they pinpoint.  They instead prefer to focus on the (correct) point that most of the mainstream approaches did not forecast the collapse in deaths and hospitalizations found in England, Sweden, New York, and now parts of the American South.  In reality, you need to put both sides of the picture together, and grasp both the insights and limitations of the herd immunity theorists.

So herd immunity does seem to be fragile, and if other developments (treatment, antivirals, steroids, masks and thus lower dosage)  lower death rates, bravo, but case behavior still moves against the simple herd immunity theory, at least in Madrid.  How fragile we still do not know, and I readily grant and indeed would emphasize that Madrid is the only major counterexample to date.  Appreciate the limits of knowledge!

If you listen to Ivor Cummins, a darling of the herd immunity theorists, he doesn’t seem to grasp these problems of possible temporariness (he loves to switch to talk of deaths at just the wrong time), but rather treats herd immunity as “it’s over,” with a few vague qualifiers tossed in at the very end.  We will see.

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