Why herd immunity is worth less than you might think

That is the topic of my latest Bloomberg column.  The evidence in favor of at least partial herd immunity continues to pile up, but still don’t get too cheery.  One worry is that herd immunity might prove only temporary:

First, many herd immunity hypotheses invoke the idea of “superspreaders” — that a relatively small number of people account for a disproportionate amount of the contagion. Perhaps it is the bartenders, church choir singers and bus drivers who spread the virus to so many others early on in the pandemic. Now that those groups have been exposed to a high degree and have acquired immunity, it might be much harder to distribute the virus.

That logic makes some sense except for one issue: namely, that the identities of potential superspreaders can change over time. For instance, perhaps choir singers were superspreaders earlier in the winter, but with most choral singing shut down, maybe TSA security guards are the new superspreaders. After all, air travel has been rising steadily. Or the onset of winter and colder weather might make waiters a new set of superspreaders, as more people dine inside.

In other words, herd immunity might be a temporary state of affairs. The very economic and social changes brought by the virus may induce a rotation of potential superspreaders, thereby undoing some of the acquired protection.

In other words, the fight never quite ends.  Here is another and possibly larger worry:

Another problem is global in nature and could prove very severe indeed. One possible motivation for the herd immunity hypothesis is that a significant chunk of the population already had been exposed to related coronaviruses, thereby giving it partial immunity to Covid-19. In essence, that “reservoir” of protected individuals has helped to slow or stop the spread of the virus sooner than might have been expected.

There is a catch, however. If true, that hypothesis means that the virus spreads all the more rapidly among groups with little or no protection. (Technically, if R = 2.5, but say 50% of the core population has protection, there is an R of something like 5 for the unprotected population, to get the aggregate R to 2.5.) So if some parts of the world enjoy less protection from cross-immunities, Covid-19 is likely to ravage them all the more — and very rapidly at that.

Again, this is all in the realm of the hypothetical. But that scenario might help explain the severe Covid-19 toll in much of Latin America, and possibly in India and South Africa. Herd immunity, as a general concept, could mean a more dangerous virus for some areas and population subgroups.

There are further arguments at the link.

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