How the CDC and WHO missed that Covid was airborne for so long

This is a fascinating article about how a “known fact” about how respiratory droplets behave was wrong, and what it took to get the WHO and CDC and medical establishment to acknowledge it, while it was costing lives.

Very early in the pandemic it was quite evident to me–as someone who is not an epidemiologist but can read a textbook on the topic, create a demographic SIR model, and draw some basic conclusions from the results–that given the rate of ‘community spread’ without any known contacts, and particularly how rapidly it moved through enclosed spaces such as cruise liners and health care facilities, that it almost had to be capable of airborne spread. Even before we had nationwide ‘data’ (and never of good, verified quality), the naive R0 was clearly >4 and potentially higher than 6 even while the average of the ‘official’ estimates was barely breaking 2. Of course, later in the May/June timeframe, several papers were published establishing R0 to be between 3.8 and 8.9, which was entirely consistent with early data, and still you had people arguing that we might be able to hit herd immunity threshold at less than 50% infected even though for those values herd immunity woudl require between 74% and 89% of the population to be inoculated through previous infection or vaccination.

Why the public health screwups in messaging even though the science and data were relatively clear (and many virologists and immunologists like those on the This Week in Virology podcast noted openly and repeatedly)? I think there was both a concern about over-alerting the public and the political pressure to try to look for the most optimistic ‘spin’ for economic reasons regardless of the consequences, so better to downplay what the data is actually pointing to until things are unequivocally worse. (My more cynical instinct is to suggest that they knew it would actually be worse but that would be someone else’s problem when it happened.)

It is certainly true that the general public (and the politicians they elect) know next to nothing about infectious disease and are largely informed by bad entertainments in which infection turns people into flesh-eating zombies, but there was a severe lack of any planning or coordination even in states where the leadership took the public health threat seriously. In California, for instance, outdoor mask mandates were broadly applied (and still remain in place despite CDC guidance) even after it was clear from data in the summer that outdoor contact, even in large groups, rarely resulted in wide contagion. Los Angeles County went even further, prohibiting outdoor dining for months despite the fact that not only was it clear that open air dining with good separation was relatively safe, but also offered the opportunity to educate the public on safe contact protocols by direct illustration, instead preferring the “Better Safe Than Sorry” strategy that people, primed by months of already being constrained and showered with contradictory instructions, started to ignore en masse for Thanksgiving and Christmas holidays.

I think large institutions, long used to operating to official dogma, find it very hard to give voice to essential truths that challenge that dogma. In the case of SARS-CoV-2, it was “just a respiratory virus” which can only spread by droplets and therefore isn’t airborne, just like influenza (even though that has long been known to be not strictly true). It was notable that Dr. Fauci spoke out against such assumptions (and has done so throughout his long career starting with the HIV/AIDS pandemic) but it is also notable how few did so even when ‘leaders’ were offering nonsensical and even sometimes harmful guidance. The WHO, in particular, seems very concerned about the politics of offending any major parties and in particular China and Russia; and perhaps understandably so for how delicate their funding situation could be without broad support, but it also makes them an unreliable broker when it comes to information critical about the handling of the pandemic.

We’re not use to handling global pandemics because it has been “so long” since we’ve had one (not counting the several influenza pandemics that fortunately have not been as virulent as the 1918 Spanish Flu, or HIV/AIDS which is not easily transmissible outside of intimate contact, sharing needles, or contaminated blood transfusions) and so there is little guidance or protocols in place despite the fact that epidemiologists have been warning for decades that this was coming. And SARS-CoV-2 was a love tap compared to what a really virulent avian influenza or some kind of novel Poxviridae outbreak could be like with a high R0 and an IFR > 5%. We really need to start treating infectious disease with the same seriousness we treat international terrorism, and I don’t mean by drone-bombing random wedding parties and extraordinarily renditioning people who kind of match a name on a list, but devoting a budget of tens of billions of dollars a year on both preparation and international surveillance for the next pandemic.


I don’t think it’s about the pandemic, especially. I think this is a fairly common pattern that we sometimes forget that even science is prone to. Where something takes hold as “common knowledge” that almost requires no citation, but, following it back, you find that there actually is no proof for it. And the scientists who try to point it out are often not listened to or even ridiculed for not “knowing” such a basic fact.

And Fauci didn’t behave any differently, as far as I can see.

Eh. Reading that article didn’t leave me with the same response. They defined aerosols as droplets of a certain size, and this definition didn’t fit COVID-19. They said that things that didn’t qualify as aerosols could still hang in the air, but for not as long. They fairly early on made it clear that the six foot rule was not sufficient on its own, and requires masks. It sure seems to me they weren’t that far wrong, and this was more a smaller technical change.

I’m fine that they decided that they needed to change their definition of aerosols to be more in line with what people expected it to me. And the story about how the definition came about is interesting to me. But I found it a long article that focused too much on the heart strings rather than the data.

I get why they do that—showing the human element of science can help it resonate. . But, for me, it feels overwrought. For science stuff where there are facts I need to know, give me the old inverted pyramid style any day.

The definition of airborne transmission is not trivial and there is still debate to this day on whether SARS-CoV-2 or even influenza viruses are airborne. The definitions of droplets vs. aerosols are also sketchy. Clearly, inhaling aerosols are going to get a virus deeper into the lungs. However, you could get Covid by simply infecting cells in your nostrils.

I’m not sure where R0 of greater than 4 are coming from. I think the R0 of SARS-CoV-2 is closer to 2.5 or at least somewhere between 2 and 4. That’s not a particularly high R0. The variants are higher but that has nothing to do with airborne transmission. My point is that R0’s are extremely high for truly airborne transmissible viruses like measles.

R0’s don’t simply depend on the mode of transmission. Infection depends on points of entry into the body and cells and how a virus can manipulate the immune system. R0’s also depend on how long someone is contagious and whether or not they are transmitting through asymptomatic means.

Physical studies on aerosols demonstrate that small droplets and evaporated viral particles can float in the air, sometimes for hours. That does not mean that those particles are infectious. Some early studies detected viral RNA in hospital vents, etc. But that does not prove that they could infect someone. Later studies involved trying to culture the viruses, but obviously, that adds a lot of error.

The best studies are epidemiological, which do demonstrate airborne transmission. However, these are usually superspreader events which include many factors besides airborne particles like viral load, ventilation, and activities. From my readings, most the aerosol particle transmission was still occurring in close proximity. Wearing masks dealt with it.

So the CDC and WHO may have been too conservative (in my opinion), but being “wrong” depends on your perspective.

Here’s a good article on this. Why airborne transmission hasn't been conclusive in case of COVID-19? An atmospheric science perspective - ScienceDirect

“High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2”, Steven Sanche, Yen Ting Lin, Chonggang Xu, Ethan Romero-Severson, Nick Hengartner, and Ruian Ke, Emerging Infectious Diseases, Volume 26, Number 7—July 2020


Severe acute respiratory syndrome coronavirus 2 is the causative agent of the ongoing coronavirus disease pandemic. Initial estimates of the early dynamics of the outbreak in Wuhan, China, suggested a doubling time of the number of infected persons of 6–7 days and a basic reproductive number (R0) of 2.2–2.7. We collected extensive individual case reports across China and estimated key epidemiologic parameters, including the incubation period (4.2 days). We then designed 2 mathematical modeling approaches to infer the outbreak dynamics in Wuhan by using high-resolution domestic travel and infection data. Results show that the doubling time early in the epidemic in Wuhan was 2.3–3.3 days. Assuming a serial interval of 6–9 days, we calculated a median R0 value of 5.7 (95% CI 3.8–8.9). We further show that active surveillance, contact tracing, quarantine, and early strong social distancing efforts are needed to stop transmission of the virus.


Well, they were dead wrong about saying that droplets bigger than 5 microns couldn’t travel more than 6 feet, which was a big part of why they wouldn’t listen to people pointing out that it was apparent that there were some cases of airborne transmission.

And the point of trying to get them to recognize it was to get more accurate information out, and more precautions in place, like wearing masks, and paying attention to ventilation systems.

But they were adamant that what aerosol experts, physicists, were telling them about how the respiratory particles moved in air was false, based on basically a myth.

Morawska had spent more than two decades advising a different branch of the WHO on the impacts of air pollution. When it came to flecks of soot and ash belched out by smokestacks and tailpipes, the organization readily accepted the physics she was describing—that particles of many sizes can hang aloft, travel far, and be inhaled. Now, though, the WHO’s advisers seemed to be saying those same laws didn’t apply to virus-laced respiratory particles. To them, the word airborne only applied to particles smaller than 5 microns. Trapped in their group-specific jargon, the two camps on Zoom literally couldn’t understand one another.

The 5 micron limit was off by a factor of 20, and yet they clung to it and just rejected what they were being told by scientists in the relevant field, because it contradicted their “knowledge” of that “fact.”

No one is saying Covid is as infectious as measles. They were trying to get WHO and CDC to acknowledge a basic fact about the disease – to stop actively calling it a myth – and to change guidance to reflect that fact.

Thanks. So they estimate shorter serial intervals and doubling times than the other researchers.

I completely agree. Did you see where they got the magic 5 micron number? It was work by Wells in the early 20th century. It was the optimal size of infectious aerosols for TB. In other words, the bacterium can only infect cells deep in the respiratory tract. You need to have particles smaller than 5 microns to be inhaled that deeply. It has nothing to do with the definition of an aerosol. Wells even made a plot of what size particles will evaporate to become floating aerosols before they fall to the ground. The size was 100 microns, which is the size Prather found out over a half a century later. LOL.

Yes, I read the article I posted, which has that in it.

I want to “like” this post.