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.
A related issue is that the regions with herd immunity are vulnerable to “invasion” from less protected parts of the country. Maybe hard-hit New York City does have partial herd immunity, but to the extent the city appears safe for months on end, outsiders are going to start to visit and live there, again undoing some of the current level of protection. Meanwhile, New Yorkers themselves will take more chances and be exposed to higher doses of the virus. Their current levels of immunity, while useful, will not make them invulnerable against all possible forms and degrees of exposure.
Once again, there is no guarantee of permanence when it comes to herd immunity.
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.
Continued monitoring of Sweden, southeast England and New York City will probably reveal whether the herd immunity hypothesis is true. But no matter how that evidence turns out, this is no time to let down our guard.