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Cases vs. deaths in the Covid debates

Summary:
Once upon a time, there were some herd immunity theorists.  They claimed that once a certain percentage of the population had been infected, the R for Covid would fall below one and the disease would become far less common and less significant.  Since these analysts were especially aware of heterogeneity issues (though common in the broader scholarly literature), these same herd immunity theorists tended to be less pessimistic than many of the mainstream forecasts. To be clear, everyone knew that herd immunity was a general and universally accepted concept in the literature.  But these particular herd immunity theorists were the ones saying it really would matter, and they did so in the bold and fearless manner.  As I mentioned earlier, the NYT didn’t really start covering this issue until

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Once upon a time, there were some herd immunity theorists.  They claimed that once a certain percentage of the population had been infected, the R for Covid would fall below one and the disease would become far less common and less significant.  Since these analysts were especially aware of heterogeneity issues (though common in the broader scholarly literature), these same herd immunity theorists tended to be less pessimistic than many of the mainstream forecasts.

To be clear, everyone knew that herd immunity was a general and universally accepted concept in the literature.  But these particular herd immunity theorists were the ones saying it really would matter, and they did so in the bold and fearless manner.  As I mentioned earlier, the NYT didn’t really start covering this issue until this August, a kind of unbelievable (and appalling) communications failure from public health experts who didn’t want to say anything that might be construed as minimizing expected risk.

Now, I don’t recall many of those theorists early on making a prediction about a specific number required for the herd immunity threshold to be reached.  Nonetheless, when deaths and hospitalizations collapsed in Sweden, London, and New York at about 20 percent seroprevalence, obviously it seemed that might be the critical level for herd immunity to kick in. (Higher measured levels of seroprevalence, such as for the slums of Mumbai might just come from the speed of ripping through a very dense and exposed community.)  And a lot of the observed later waves were in fact coming in other parts of these countries or regions, such as Barcelona following Madrid, or Arizona following New York.

These herd immunity theorists were correct in predicting an “earlier than the mainstream is telling you” collapse in deaths and hospitalizations in the hard hit regions.  And that is very much to their credit.

You will note that part of their prediction or implied prediction was that past the herd immunity point cases should fall, not just deaths.  Transmission just would not be very effective or speedy any more, so cases should be low whether or not people die in the hospitals or the hospitals can save them.  I’ll be coming back to this.

Then things started to go askew in the last few weeks.  First, it seems like a bad second wave came to an already fairly hard hit Madrid.  OK, you could say Madrid was never had 20% seroprevalence to begin with.  And then what appears to be a second wave has started coming to Israel, with rising hospitalizations.  Finally, it is believed that in Britian R equals about 1.7, and that a second wave of cases is on the verge of hitting London and Southeast England.  That hasn’t quite happened yet, but the informed authorities greatly fear it, and the numbers so far seem to indicate that as the trend.

Added all up, those data points are not decisive in rejecting the claims of these herd immunity theorists.  But they do make the herd immunity theorists look less correct than they did say three weeks ago.  Those “partial second waves,” or whatever they turn out to be, seem more active than one might have expected.  Again, though, the story is still unfolding and we should not rush to final conclusions.  But in the meantime we should update!

In response, many of the herd immunity theorists strike back and ask “where are the deaths“?  But that is not the right question for testing herd immunity claims.  Those claims were about transmission slowing down, and those claims should be true about Covid-19 cases whether or not more people are surviving in the hospital.  (Imagine for instance a perfect antiviral that saved everybody — would that mean herd immunity was true a priori?  Nope.)

Another claim from some of the less careful herd immunity theorists is that cases are rising again because testing is rising.  That doesn’t seem to explain observed patterns in Israel, Spain, or England, where in all instances actual Covid cases are rising above and beyond what is going on with testing policy, and by some considerable margin.  It probably does explain some parts of America, however.

It is very likely that death rates will be much lower this time around, because of better procedures, younger victims, lower doses, and possible (speculative!) variolation through mask use over time, exposing people to lower doses repeatedly and boosting their immune responses.

There is a temptation to say “few deaths, we don’t need lockdowns!”  Indeed, the more partisan of the herd immunity theorists are obsessed with the lockdown issue.  Lockdowns are important questions, but don’t let your lockdown views skew your interpretation of the numbers, and furthermore there are many other important Covid questions of interest, for instance:

1. How much more should we invest in better hospital procedures?  Better biomedical fixes?  And how much should we hurry?  If transmission is mostly over, you can relax much more, but ongoing cases both will bring some long-term damages (short of death) and also some ongoing panic, whether rational or not.

2. How do we deal with the fact that case numbers per se will scare people for a long time to come?  Again, if transmission is winding down, you don’t need as big a long-term plan here.

3. Should you let large swarms of tourists into your currently semi-protected region, say it is Venice, Italy or the less infested parts of Hawaii?

4. To the extent there is current herd immunity or semi-herd immunity as I call it, how fragile is that arrangement with respect to a possible rotation of potential super-spreaders?  And what might set off those fragilities?

For those questions, and indeed many others, it matters a great deal whether the original herd immunity prediction about “permanently low cases past the herd immunity threshold” is correct, or not.  Whether the death rate is high or low.  You really do need to understand about the cases in their own right, once you see this broader spectrum of issues at stake.

The more partisan herd immunity theorists wish to debate “how terrible will this be and will that justify a lockdown?”, and then they seek to talk you into a mood of not being so terrified, because frequently they are lockdown skeptics.  Again, that is a super-important question.  But don’t let it distract you from the other important questions at hand.

And for those other questions, as I’ve already stated above, the trajectory caseload predictions of the herd immunity theorists are looking worse than they did a few weeks ago.

Of course I will be giving you updates on this matter as time passes.  But this is the very latest, namely that some of the herd immunity theorists are on the precipice of being dogmatically wrong about matters of real import, just as were some of the most pessimistic mainstream predictions from March and April.

The post Cases vs. deaths in the Covid debates appeared first on Marginal REVOLUTION.

Tyler Cowen
Tyler Cowen is an American economist, academic, and writer. He occupies the Holbert C. Harris Chair of economics as a professor at George Mason University and is co-author, with Alex Tabarrok, of the popular economics blog Marginal Revolution. Cowen and Tabarrok have also ventured into online education by starting Marginal Revolution University. He currently writes the "Economic Scene" column for the New York Times, and he also writes for such publications as The New Republic, the Wall Street Journal, Forbes, Newsweek, and the Wilson Quarterly.

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