Post #771: Aerosol transmission of COVID-19, revisited. Why won’t the CDC change its position?

Posted on August 6, 2020

Source of illustrations below: COMMENTARY: COVID-19 transmission messages should hinge on science. March 16, 2020, Lisa Brosseau, ScD, author, on line at the University of Minnesota Center for Infectious Disease Research and Policy.  Used without permission here, under a claim of fair use.

To cut to the chase, I think there would be some extreme political fallout if the CDC changed its position on aerosol transmission of COVID-19.  And that — not scientific evidence — is why the CDC is not going to say that aerosol transmission of COVID-19 is a significant problem.  The rest of this posting just lays out the reasoning behind that assertion.


I keep returning to the topic of aerosol (airborne) spread of COVID-19.  And to the fact that the World Health Organization (WHO) and the US Centers for Disease Control (CDC) absolutely will not acknowledge that aerosol spread is a material issue.

Just yesterday, a study was published suggesting that aerosol (airborne) transmission played a major role in COVID-19 spread aboard the Diamond Princess cruise ship.  That, as reported by the NY Times, or you can read exactly what the researchers wrote (although it’s a pretty hard slog if you’re not in this line of business.)  Key finding (emphasis mine):

Moreover, close-range and long-range transmission likely contributed similarly to disease progression aboard the ship, with fomite transmission playing a smaller role.

Source:  Mechanistic Transmission Modeling of COVID-19 on the Diamond Princess Cruise Ship Demonstrates the Importance of Aerosol Transmission, Parham Azimi, Zahra Keshavarz, Jose Guillermo Cedeno Laurent, Brent R. Stephens, Joseph G. Allen,

Admittedly, that’s a simulation (not an experiment), and that’s a pre-print (not yet accepted for publication), but it looks like a credible source, to me.  Plus, we’ve had enough “superspreader” events now that we know the drill.  What I found particularly useful with this most recent research is the quantification:  Roughly equal parts droplet and aerosol transmission.

To rephrase that:  If you ignore aerosol transmission of COVID-19, you’re missing half the picture. At least, by that estimate, in that setting above.

To be clear, I’m hardly the only person to have expressed frustration with the unwillingness of WHO and CDC to change their position.  Many qualified scientists have been asking those those organizations to do so.  This, as reported by the NY Times, or, if you wish, you can read exactly what they wrote.

That brief commentary (signed by hundreds) is so good, let me place a copy here, for your convenience.  Just the first few paragraphs lay out the case in a hard-to-ignore, point-after-point-after-point style.  No need for me to elaborate.  They tell you what the evidence is, and why this matters for policy.



Finally, while this is the most recent summary of evidence, take a look at the date of the very first reference cited above — the source for my illustrations.  Smart and qualified academics were raising this point, politely, months ago.  So it’s not like this group of scientists suddenly lost it and published this in a fit of pique.  The fact that CDC ignores aerosol transmission of COVID-19 has been an issue of scientific concern for a considerable length of time.

But why won’t the CDC acknowledge it?

One of my wife’s friends raised an interesting question:  Why won’t the CDC acknowledge the importance of aerosol transmission?  Here, I’m going to give my best answers to that.  This section is, of course, pure speculation.  The only organization that can truly answer this is the CDC itself.

1:  They don’t think the evidence supports it.

First, from what I have read, I believe that some CDC staff really and truly think that aerosol transmission is rare, based on their contact-tracing evidence, and what they think they see regarding patterns of transmission.  In other words, as CDC staff reads the numbers, close contact (within droplet range) provides an adequate explanation of disease spread.

I am absolutely unable to understand their reasoning on this, because, in so far as their public-facing data are concerned, when they decided that droplet transmission explained everything, they categorized all instances of transmission into just three buckets, as shown below.  Note that there’s no bucket for “aerosol spread”, or even a bucket for “community transmission” (see below).  At that time, the CDC’s methods allowed for three cases:  Travel, close contact, and case still under investigation.  How they ruled out aerosol spread, based on these data, is beyond me.

Source:  CDC

The second reason I think that contact tracing/pattern of spread evidence isn’t helpful here is the frequent occurrence of “community spread”.  Community spread is a technical term, and it means that, after investigation, public health officials could not determine how a person had gotten infected.  If that were rare, you might be able to make some strong inference about how the disease spreads.  But, again per the CDC:

Source:  CDC

So, as of a couple of months ago, “community transmission” was widespread almost everywhere.  In other words, almost every state was coming up with numerous cases for which there was no obvious point of infection.

In fairness, aerosol transmission if COVID-19 looks nothing like aerosol transmission of measles.  Measles is incredibly infectious, COVID-19 is far less so.  But, e.g., tuberculosis is also far less infectious than measles, and aerosol spread is a concern for that disease.

So, how, exactly, CDC staff think that the pattern of spread of this disease rules out aerosol transmission, I can’t quite grasp.  Clearly, with aerosol spread, it’s all a matter of dosage.  In an enclosed hospital room, with very ill patients, health care workers wear N95 respirators specifically due to the threat of aerosol spread.  Outdoors, likelihood of transmission of disease is vastly lower than indoors.  But to go from that first situation (hospital room) to the second (outdoors), and assert that nowhere in-between is the aerosol dosage high enough — that’s just not supported by any logic that I can fathom.

2:  They don’t like the consequences of changing their position.

This section is purely speculation.  But it’s still worth writing down all the consequences if the CDC admits that aerosol transmission is a material risk in community (non-hospital) settings.

First, there’s just plain-old human nature.  Nobody wants to admit being wrong. And to admit that droplet transmission does not explain all the cases of infection would be a major turnaround for the CDC.

Second, it would mean that the CDC’s initial guidance could be characterized has having been disastrously incorrect.  The CDC, recall, initially said that social distancing (and hand washing) was all that was needed.  In particular, the CDC specifically recommended social distancing and no mask use.  And that made logical sense if and only if droplet transmission was the only significant means of transmission.  As long as you stayed out of range of an infected person’s droplets, you’d be safe.

But if aerosol transmission is as common as droplet transmission, then a) social distancing alone won’t stop spread, and b) everybody should be wearing the best mask they can get, when indoors.  And should be wearing that in addition to keeping proper social distancing.

Third, it would throw a major monkey wrench into re-opening plans.  Any situation where you are indoors bears some risk unless you wear a mask capable of block most or all aerosols.  Any indoor situation with crowds and poor ventilation creates risk, social distancing or not.  And any situation like indoor dining — where you can’t be masked — has inherent risks that cannot be solved simply by keeping tables 6′ apart.  And I don’t even want to think about the implications for re-opening schools, except to say that if we do that, we should feel some obligation to provide high-quality (N95) masks to teachers.

Ideally, buildings involved in re-opening would have to have their ventilation systems inspected and possibly modified.  The idea being that in modern energy-efficient buildings, you bring in as little fresh air as possible.  (In fact, many systems for ventilating meeting spaces respond to carbon dioxide buildup, and do not introduce fresh air until it gets “stuffy”, defined by LEED standards as 1200 PPM C02).  In the COVID-19 era, by contrast, you’d want to have as much fresh air exchange as possible.  Modern, tightly-sealed, energy-efficient buildings are exactly what you don’t want when a disease is spread by aerosol (airborne) transmission.

Fourth, it would justify purchase of N95 respirators by the general public.  And the CDC wants to play its part in keeping those out of the hands of citizens, and keeping those reserved solely for health-care workers.  But if aerosol transmission matters in the community setting, then everybody has a justification for wanting some face covering that efficiently blocks aerosol-sized particles.

In particular, the CDC’s guidance for citizens to wear a “cloth mask” is pure social engineering, not science.  The right guidance would have been to wear the best mask they could get.  But the CDC wanted to suppress citizen attempts to purchase N95 respirators.  Hence, “cloth”.

And so, if the CDC says that aerosol transmission is real, the CDC’s current explicit guidance to use “cloth masks” can be viewed as endangering citizens.  Cloth masks filter some significant fraction of aerosols, but they are nowhere near as good as proper N95 respirators.  By telling US citizens to wear cloth masks (instead of just saying “masks”), the CDC specifically directed us to adopt a risker-than-necessary approach (cloth, instead of N95 if available).

Finally, given the above, retribution from the President would seem to be a certainty if the CDC overtly changes its position on aerosol transmission of COVID-19.  So far, the President’s strategy seems to be almost exclusively one of wishful thinking combined with blame-shifting (e.g., de-funding the WHO in the midst of the worst pandemic in a century; blaming current problems on the Obama administration.)  Imagine what would happen if the CDC now came out and said “our initial advice was wrong AND it will be vastly harder to re-open certain types of businesses and schools.”  As long as the President’s appointees are running DHHS, it’s a pretty fair bet that the CDC is not going to be able to say that aerosol transmission is important.

What we seem to have, instead, is the CDC offering some guidance that only makes sense if aerosol transmission matters materially.  But not actually saying that aerosol transmission matters.  E.g., any guidance related to using outdoor settings, to maintaining high levels of ventilation, or to wearing masks when indoors — these all make sense if and only if aerosol transmission matters.  (If it’s all droplet transmission, then the original CDC guidance of social distancing, no masks, and wash you hands would be adequate.  And outdoors would be no safer than indoors).

In summary, I think the CDC won’t change its position on aerosol transmission.  I don’t think that’s based on any firm evidence.  To the contrary, I think they maintain that only by turning a blind eye to a considerable body of evidence.  I think that’s based on all of the negative consequences for CDC and CDC staff if they do change their position.

Essentially, it’s a political decision, not a scientific decision.  And that’s why we have ended up with CDC guidance that’s such a garbled muddle.  Some, but not all, of the guidance only makes sense if aerosol transmission matters.

And that’s why we have a large block of scientists signing a letter to try to pressure the CDC to change, as noted above.  That letter isn’t really addressed to the scientific community.  Scientists can read the evidence on their own.  It’s addressed to the politicians who ultimately are in control of the US DHHS and the CDC.  It’s an attempt to get policy makers to base their policy on the science, and not on the politics.  I wish them the best of luck with that.