CDC Flip-Flops on Paradigm Shift to SARS-COV-2 Transmission via Aerosols

By Lambert Strether of Corrente.

Patient readers, I’m a little pressed temporally, and so this post may be a little less coherent than usual. Let me jump in–

As readers know, I stan for aerosol transmission as the primary tranmission mechanism for SARS-COV-2; that is, singing, shouting, talking, even breathing, all of which give rise to small virus-bearing particles that float indefinite distances (aerosols), as opposed to coughing or sneezing, which give rise to larger particles that fall, pulled down by gravity (droplets), after travelling one or two meters (and also accumulate on surfaces, which are then to be wiped). As I wrote back in May:

From the beginning of the #COVID19 pandemic, we’ve been washing our hands, masking up, cleaning surfaces, and social distancing. These measures have worked (especially masking), but now we know more. There’s mounting evidence that airborne transmission indoors is a key — perhaps the main — pathway to SARS-COV-2 transmission. In this post I want to look at why that’s so, give examples, and suggest a simple heuristic to stay safe. Material like this might also be used to inform public policy (here; here) by reducing superspreader events in enclosed spaces like churches (airborne transmission via singing), restaurants (loud talking, especially if room is noisy), bars (ditto), nursing homes (shouting[1]), gyms (grunting), meat-packing plants (shouting), call centers (talking), offices generally (air conditioning), and other hot spots, but working that policy out is not the object of this post (see here for engineering controls for airborne transmission, and here for covid-proofing public spaces).

I took this view because of case studies, given in the post, for which aerosol transmission could give an account, and droplet tranmission could not. (Subsequently, actual transmission of viable viral material through the air was demonstrated in two hospital studies, posted in Links.)

So it was with great interest that I received the following mail from alert reader Chi Gal in Carolina, saying that the CDC had finally updated its guidance to support aerosol transmission. (I quote Chi Gal to give her a hat tip for taking point on this topic in comments). Chi Gal wrote:

Subject: Finally, the CDC updated its guidance! (link from ChiGal).

On September 18, just 7 months in. Maybe now we have a shot at getting this thing under control.

First, I checked the Twitter, and found this headline from the Los Angeles Times: “CDC says coronavirus spreads mainly in the air, through respiratory aerosols and droplets.” OMG! Happily and excitedly, I went to the CDC website, where this notice appeared:

I took a screenshot and sent it to Chi Gal:

“Did the version you saw have this at the top:”


And that, dear readers, is all that anybody knows (at least out here in the Great Unwashed). Here is the CDC guidance that appeared and disappeared. From the Hill, the CDC guidance that appeared and disappeared:

“There is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet (for example, during choir practice, in restaurants, or in fitness classes),” the agency had written. “In general, indoor environments without good ventilation increase this risk.”

“These particles can be inhaled into the nose, mouth, airways, and lungs and cause infection,” the deleted guidance said. “This is thought to be the main way the virus spreads.”

OMG, indeed.

* * *

So what happened? I can come up with two possible accounts: The institutional and the political. (Of course, “the institutional is the political,” as Carol Hanisch did not say, but let us keep the two separate, if only as objects of study).

Institutionally, the conflict between proponents of droplet transmission, and the proponents of aerosol transmission, is part of the process known in the history of science as a paradigm shift, as discussed here in “Don’t ‘Trust the Science,’ Trust Science While You Hone Your Critical Thinking Skills.” (Here is an example at NC of a paradigm shift in river restoration). This excellent thread from aerosol scientist Jose-Luis Jimenez shows such a shift in action. It’s worth reading in full, but I’ll pull out the key tweets. Jimenez begins by critiquing the now-vanished CDC guidance:

(His basic point, explained over several tweets, is that CDC cannot, at the same time, claim that Covid spreads by “inhalation” of droplets, because droplets are too big. (I mean, if I hack up a loogie, you’re not inhaling it, right? And so on down to the smallest possible size of droplet.) But here is the key point:

And why does Chapin rule out airborne infection?

“A great relief to most persons to be freed from the specter of infected air” means “people might panic” (!). And here we are:

You can read the rest of the thread for today’s institutional players: Basically, WHO and the CDC people have been droplet proponents for a long time, and, since science proceeds by conflict — which is why “Trust the science!” doesn’t work when applied uncritically — they need to be persuaded, or, if worst comes to worst, defenestrated in the usual way: One funeral at a time, as Max Planck said. To be fair to the medical profession, they have proceeded with far greater dispatch than physicists!

So, one explanation for the new CDC guidance being pulled is that, institutionally, the old guard won.

Politically, you know the already congealing narrative. Here is the classiest, least hysterical example of it.

But there are a few problems with Mitchell’s Tweet.

First, Democrat operative and medical ethicist[1] slash private equity/venture capitalist partner Zeke Emanuel is eminently quotable but foolish: As we have explained at length, “the science” is not always a matter of “facts” but of the paradigms we use to give an account of facts. WHO, for example, does not regard aerosols as the primary transmission path for Covid as a fact at all. Zeke’s embarrassing Neera on this, and he should do better.

Second, I have checked CBS, The Hill, NPR, Politico, WaPo, a second article in WaPo, and the Wall Street Journal. None of them suggest the guidance was “squelched” for “political considerations.” The New York Times says explicitly that it wasn’t:

Experts with knowledge of the incident said on Monday that the latest reversal appeared to be a genuine mistake in the agency’s scientific review process, rather than the result of political meddling. Officials said the agency would soon publish revised guidance.

Of course, one never knows when the blow may fall; anonymous sources could contradict the Times tomorrow. Nevertheless, Occam’s Razor would suggest than when we have an institutional account, we don’t need to invent a political one.

Third, and ironically, if there was, anybody doing the squelching — in today’s impoverished analytical environment — would be able to say “I did trust the science! I checked with WHO!!”

So that is the state of play on the CDC’s aerosol guidance as of today. Let’s see what they come up with!


[1] It would be unfair to call Emanuel a homicidal maniac based on a single article entitled “Why I hope to die at 75,” even if from today’s Water Cooler we note that Japanese master painter and woodblock printer Hokusai “understood the structure of animals, birds, insects and fishes, and the life of grasses and plants” only at age 73, dying 16 years later at the age of 89.

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