Post #889: The upside of the current COVID situation in the Dakotas: Herd immunity by New Year’s Day.

Posted on November 13, 2020

I’m the sort of guy who knows, deep in my heart, that behind every silver lining, there’s a dark cloud.  I’m going outside my comfort zone now to point out one clear upside to the high new COVID-19 infection rate in the Dakotas.  If they keep this up, they’ll be the first state in the US to achieve herd immunity, and they’ll plausibly do that before the end of the year.

You think you’ve made something idiot proof, then society makes a better idiot.

Source:  Loosely adapted from this Reddit for system administrators.

Herd immunity is notion that infectious diseases won’t spread, once a large enough fraction of the population is immune to them.  You don’t need to have everyone immune.  You just have to have enough of the herd develop immunity that the disease finds fewer and fewer targets, until at some point there aren’t enough to continue the chains of infection, and the epidemic or pandemic dies off.

Herd immunity is what allows anti-vaccination parents to do their thing and yet not see their children die from various childhood communicable diseases.  Their children are protected — not by faith, luck, or magic — but by being surrounded by a herd of vaccinated children.  Those vaccinated children stand between any source of infection and an unvaccinated child, breaking the chain of disease transmission.  The unvaccinated child is protected, not as an individual, but by being part of the (vaccinated) herd.

In prior posts, I’ve ridiculed people who were pushing herd immunity as a strategy for dealing with COVID-19.  Mostly, they appeared to be too dumb to do basic grade-school arithmetic.  That is, the arithmetic of just how many years it would take to achieve herd immunity, by letting the infection spread at (then) current rates.  (In fact, I used the then-current rate in Virginia.)  Versus how long we were likely to have to wait for a vaccine.  I did that most recently in Post #870.

The people pushing a herd immunity strategy just didn’t quite seem to realize where we were, in this pandemic.

I thought that was a fairly idiot-proof argument.  Take the population not yet infected, take the current rate of infection, and it will be obvious under any reasonable assumptions that it will be years before you’ll achieve herd immunity.  Or, at least, achieve it in Virginia, as it then stood.

But I failed to anticipate the development of a better idiot.  In this case, I did not think a state would ever achieve a daily new COVID-19 infection rate like that of North Dakota.  I had assumed that either a) the hospital system would break down, or b) there would be some definitive policy actions taken to reduce the rate of infection.  I never anticipated seeing an infection rate of 180 individuals per 100,000 population per day, combined with a policy response that boils down to que sera sera.

So I have to redo the math.

Right now, 7.8% of North Dakota’s population is known to have been infected.  That’s 59,000 people (per this website), out of a population of about 760,000 (per the US Census Bureau).  At the current rate of new infections (180/100,000/day), they are adding to that at the rate of 1.3% per week.

The big unknown is the fraction of the population that has been infected but not tested.  That’s hard to say in the best of circumstances.  There is piecemeal and spotty information from various sources, all of which boil down to samples-of-convenience of people who are out and about and being active.  Estimates of the ratio of total infections to diagnosed infections range anywhere from about 3 to 12.  And those figures will, of course, reflect testing patterns that were prevalent half a year ago, when most of them were obtained.

So, you have to take a guess.  I’m guessing a ratio of 4-to-1.  That is, for every person known to have been infected (via PCR test), there are another three out there that were asymptomatic, or sufficiently mildly symptomatic that they did not seek medical attention or testing.

The only other piece of the puzzle is that, best guess, you need 70% to have been infected before you achieve herd immunity.

Under those assumptions, if the current rate of infection continues unabated, North Dakota will achieve herd immunity less than eight weeks from now.  Herd immunity by New Years Day!

Of course, it won’t actually work that way.  As the fraction of the population ever infected increases, potential targets for the virus start getting fewer and further between.  The rate of new infections would have to fall.  In reality, North Dakota would not run flat-out until it hit herd immunity.  It would sort of ease up to it, with the rate of new infections tapering off as they approach herd immunity.

That said, I think we all have to take our hats off to the brave people of North Dakota, for making a good solid effort to achieve herd immunity.  Their success or failure will provide some sort of lesson for the rest of the U.S.A.  I’m just not quite sure the rest of us are ready to follow in their footsteps.