Let’s talk about herd immunity.

Are some cities and regions acquiring it sooner than expected and thus have higher than expected protection against Covid-19? https://trib.al/ACe2r0n 

The evidence for herd immunity can be seen in:

Sweden, even though the Swedes rarely wear masks or social distance.
London, England, where the bars and movie theaters are open, yet the health situation appears to be stable http://trib.al/ACe2r0n 

In both London and Sweden, case and death rates have plummeted.

Some research suggests that regions acquire partial herd immunity at 20% exposure. Earlier estimates had suggested up to 70% exposure would be needed. If true, this could be very good news http://trib.al/ACe2r0n 

But there are caveats.

Many herd immunity hypotheses invoke the idea of “superspreaders.” That logic makes some sense except for one issue: namely, that the identities of potential superspreaders can change over time http://trib.al/ACe2r0n 

Perhaps choir singers were superspreaders, but with most choral singing shut down, maybe TSA security guards are the new superspreaders.

Or the onset of winter and colder weather might make waiters a new set of superspreaders, as more people dine inside http://trib.al/ACe2r0n 

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 http://trib.al/ACe2r0n 

Regions with herd immunity might be vulnerable to “invasion” from less protected parts of the country.

Maybe hard-hit New York City has partial herd immunity, but outsiders are going to start to visit, undoing some of the current level of protection http://trib.al/ACe2r0n 

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 http://trib.al/ACe2r0n 

One possible motivation for the herd hypothesis is that a chunk of the population already had been exposed to other coronaviruses, thereby giving it partial immunity to Covid-19.

This “reservoir” of protected individuals could slow the spread of the virus http://trib.al/ACe2r0n 

But if some parts of the world are less exposed to cross-immunities, Covid-19 is likely to ravage them all the more — and very rapidly at that http://trib.al/ACe2r0n 

That hypothetical scenario might help explain the severe Covid-19 toll in:

Latin America
India
South Africa

Herd immunity, as a general concept, could mean a more dangerous virus for some areas and population subgroups http://trib.al/ACe2r0n 

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 http://trib.al/ACe2r0n