CDC Recommends Masking for School Staff, Accepts Aerosol Transmission (Without Using the Word)

By Lambert Strether of Corrente

I had hoped to post on something slightly less mundane than a CDC study from Morbidity and Mortality Weekly Report, but May 21’s “Mask Use and Ventilation Improvements to Reduce COVID-19 Incidence in Elementary Schools — Georgia, November 16–December 11, 2020” really does seem to be different. This time. I apologize if the post is a little thin, but every time CDC drags its sluggish body toward the light, and not the darkness does the right thing, we should take notice and applaud.

Readers will recall that the first CDC guidance on school re-opening, released on February 12 of this year, was appallingly bad. As I wrote back on February 18[1], when the guidance was released:

You will notice immediately that aerosol transmission is omitted; the message of Strategy is that aerosol transmission can be ignored because it is not an “essential” element. This message is getting through; from an acute summary of Strategy in WaPo, “The CDC’s plan to reopen schools seems to prioritize expediency over teachers’ health“:

Here’s what the guidance does well. Unlike the Trump administration’s watered-down recommendations that were couched in language such as wearing masks “if possible,” these new guidelines are clear on many points. They lay out five mitigation measures that every school should implement: masking, distancing, hand-washing, cleaning and contact tracing. The part I like the most is how the CDC has divided schools into four categories based on degree of covid-19 transmission in their surrounding communities. At low and moderate levels, full in-person learning can occur; at higher levels, hybrid or reduced attendance is recommended.

Again, the CDC’s “mitigation strategies” omit discussion of aerosols entirely (even to ventilation). Frankly, I was gobsmacked. I hate to deploy terms like “criminal malpractice,” so I won’t, but such was my level of gobsmacked-ness. Further, if you accept, as I do, the SARS-COV-2 can be transmitted by aerosols, the CDC Transmission document — “the science” backing up the recommendations — rests on a very modest epidemiological base. Let’s consider first the CDC’s suppression of aerosol transmission.

I then tabulated the 61 sources that CDC cited for scientific backing, and discovered the following:

You will observe there are 14 epidemiological cases in all. Recalling the example of Norway above [where students played in the open air every 45 minutes], we throw out all the examples except those from the US. Four remain, which I have helpfully highlighted. Two are gyms, which are nothing like classrooms in their patterns of ventilation, both in Florida (!). One is a choir room, ditto. One remains.

Therefore, as far as school re-opening classroom epidemiology in the United States goes, transmission depends on one case. Transmission is an enormous inverted pyramid whose apex is a single point. I have to say that balancing act doesn’t fill me with confidence.

Well, May 21 – February 12 = 101 days later, the CDC has its own study. And things change. The Summary Section throws a number of bombshells, which I have helpfully numbered, (thus):

What is added by this report?

COVID-19 incidence was 37% lower in schools that required teachers and staff members to use masks and 39% lower in schools that improved ventilation(1). Ventilation strategies associated with lower school incidence included dilution methods(2) alone (35% lower incidence) or in combination with filtration methods(3) (48% lower incidence).

What are the implications for public health practice?

Mask requirements for teachers and staff members and improved ventilation are important strategies in addition to vaccination(4) of teachers and staff members that elementary schools could implement as part of a multicomponent approach to provide safer, in-person learning environments.

Bombshell (1) Masks and improved ventilation work to prevent Covid in the schools.[2]

(2) “Dilution” means “opening windows, opening doors, or using fans,” as the report says further on. Here we see CDC admitting aerosol transmission without actually using the word; obviously, since cigarette smoke is an aerosol, which floats, you open a window to get rid of the smell. But if somebody hawks up a loogie, which falls, even if droplet-sized, you don’t open the window to get rid of it (though you might wipe the area clean).

(3) “Filtration” means HEPA filters (though DIY box fan filters, which CDC does not mention, are also possible). Here again see CDC admitting aerosol transmission without actually using the word. Once again, if — heaven forfend — you or a friend are smoking cigarettes in hotel room, you might turn on the air conditioning unit in the hopes that its filter will clean up the aerosols. But if somebody coughs or sneezes, you don’t turn on the aircon to deal with the droplets!

(4) Mask requirements even with vaccination will make some teachers’ unions happy, although I do note this masking requirement does not (seem to, see below) apply to children. (Of course, there’s some wiggle room with “could implement as part of a multicomponent approach, but 37% lower incidence with masks is hard to argue with, though some will try.) I don’t know whether that makes this portion of Biden’s triumphalist Covid speech on May 13 inoperative or not:

Just a few hours ago, the Centers for Disease Control and Prevention — the CDC — announced that they are no longer recommending that fully vaccinated people need wear masks. This recommendation holds true whether you are inside or outside.

I say it does (and for good reason, since even a fully vaccinated teacher could transmit to a classroom, even if the odds are low). Now, whether what Biden said on May 13 also contradicts what CDC said on May 13 is a separate issue, one that demands I pull on my yellow waders, which I plan to do in due course.

Following its Summary Section, CDC goes into methodology, which reads in relevant part:

Using data from Georgia kindergarten through grade 5 (K–5) schools that opened for in-person learning during fall 2020, CDC and the Georgia Department of Public Health (GDPH) assessed the impact of school-level prevention strategies on incidence of COVID-19 among students and staff members before the availability of COVID-19 vaccines.† Among 169 K–5 schools that participated in a survey on prevention strategies and reported COVID-19 cases during November 16–December 11, 2020, COVID-19 incidence was 3.08 cases among students and staff members per 500 enrolled students.§ Adjusting for county-level incidence, COVID-19 incidence was 37% lower in schools that required teachers and staff members to use masks, and 39% lower in schools that improved ventilation, compared with schools that did not use these prevention strategies.

n = 169 schools seems like a pretty good number.

Finally, the CDC underlines its “multiple prevention strategies” rubric. More colorfully, this is known as the “Swiss Cheese Model“:

Now, our theory checker passes the model, but in practice the model is only as good as its layers. For example, the “hand hygiene” “slice” is directed at fomite transmission, which we now know is not primary (and leads to a lot of hygiene theatre, too). Ditto “touching your face.” And we might add a slice to measure CO2 levels, that being a good proxy for dilution (or lack thereof; “Don’t share air”). In any case, the CDC was recommending “multiple prevention strategies” back in the days when droplet transmission ruled the earth, and so, in addition to its strategic merits, the model has much to recommend it from the bureaucratic standpoint of always deploying a gluteal shield. That said:

CDC recommends implementing multiple prevention strategies (2) (e.g., physical distancing, masking, improved ventilation, and contact tracing) that have been associated with lower SARS-CoV-2 transmission in kindergarten through grade 12 settings (3–5). Since the completion of this study, COVID-19 vaccines have become widely available, and CDC recommends vaccination for teachers, staff members, and students aged ≥12 years (2). Until vaccines are available for children aged <12 years, universal and correct mask use is a critical prevention strategy CDC recommends that schools prioritize regardless of vaccination status for in-person learning (2). In the current study, the lower incidence in schools requiring mask use among teachers and staff members is consistent with research on mask effectiveness (6), and investigations that have identified school staff members as important contributors to school-based SARS-CoV-2 transmission (7).

Now, and I think controversially:

The 21% lower incidence in schools that required mask use among students was not statistically significant compared with schools where mask use was optional. This finding might be attributed to higher effectiveness of masks among adults, who are at higher risk for SARS-CoV-2 infection but might also result from differences in mask-wearing behavior among students in schools with optional requirements.

I think this translates to the idea that students in general are so undisciplined about mask use that it makes no difference either way. More:

Mask use requirements were limited in this sample; 65.1% of schools required teacher and staff member mask use and approximately one half (51.5%) required student mask use. Because universal and correct use of masks can reduce SARS-CoV-2 transmission (6) and is a relatively low-cost and easily implemented strategy, findings in this report suggest universal and correct mask use is an important COVID-19 prevention strategy in schools as part of a multicomponent approach.

[U]niversal and correct use of masks,” following a discussion of required masks use among staff and students, would seem to contradict “mask requirements for teachers and staff members” (twice repeated) in the Summary. Somebody must think it’s clear, since the paper has like a dozen authors, but it’s not clear to me.

* * *

So that is my reading of Morbidity and Mortality Weekly Report’s new study on mask use and ventilation in schools. I’m pleased to see that CDC’s scientists are coming to their senses at long last following where the epidemiology leads. If in fact, which Heaven forfend, there is a Covid uptick when the schools re-open in the Fall, we will welcome having these particular “multiple prevention strategies” in place.

I know I do harp on whether the word “aerosol” is used. However, it’s important that we use the right names for things. We need to name the theory of transmission — “aerosol transmision” — or else we’re robots simply following the rules, unable to adjust to new situations. (For example, “open the windows” is a rule, and a good one. It doesn’t tell you to think about what to do in new situations. Only a theory of transmission can do that.) More importantly — hold onto your hats, here, folks — elites never, ever, ever hold themselves accountable. See especially the Iraq War, where everybody who was wrong is still respected and in power, and everybody who was right was and is marginalized. If we say “aerosol transmission,” those who got it right are more likely to be rewarded and empowered, which is how science should work after a paradigm shift. So now, we wait to see the science translated into guidance. Over to you, Rochelle!


[1] This was the post that Google suppressed, or at least had an extended hiccup while adding it to their search index.

[2] I remain stunned at Kindly Uncle Joe’s Excellent Plexiglass Adventure, in a death trap of a closed, low-ceilinged, crowded school room with no evident fans, filters, or indeed windows, open or not. How on earth did the White House staff allow that charlie foxtrot of a photo op to happen? It would be nice if now they sent him to a properly ventilated school room and had him point to a fan or a filter or an open window or an open door. Or something.

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About Lambert Strether

Readers, I have had a correspondent characterize my views as realistic cynical. Let me briefly explain them. I believe in universal programs that provide concrete material benefits, especially to the working class. Medicare for All is the prime example, but tuition-free college and a Post Office Bank also fall under this heading. So do a Jobs Guarantee and a Debt Jubilee. Clearly, neither liberal Democrats nor conservative Republicans can deliver on such programs, because the two are different flavors of neoliberalism (“Because markets”). I don’t much care about the “ism” that delivers the benefits, although whichever one does have to put common humanity first, as opposed to markets. Could be a second FDR saving capitalism, democratic socialism leashing and collaring it, or communism razing it. I don’t much care, as long as the benefits are delivered. To me, the key issue — and this is why Medicare for All is always first with me — is the tens of thousands of excess “deaths from despair,” as described by the Case-Deaton study, and other recent studies. That enormous body count makes Medicare for All, at the very least, a moral and strategic imperative. And that level of suffering and organic damage makes the concerns of identity politics — even the worthy fight to help the refugees Bush, Obama, and Clinton’s wars created — bright shiny objects by comparison. Hence my frustration with the news flow — currently in my view the swirling intersection of two, separate Shock Doctrine campaigns, one by the Administration, and the other by out-of-power liberals and their allies in the State and in the press — a news flow that constantly forces me to focus on matters that I regard as of secondary importance to the excess deaths. What kind of political economy is it that halts or even reverses the increases in life expectancy that civilized societies have achieved? I am also very hopeful that the continuing destruction of both party establishments will open the space for voices supporting programs similar to those I have listed; let’s call such voices “the left.” Volatility creates opportunity, especially if the Democrat establishment, which puts markets first and opposes all such programs, isn’t allowed to get back into the saddle. Eyes on the prize! I love the tactical level, and secretly love even the horse race, since I’ve been blogging about it daily for fourteen years, but everything I write has this perspective at the back of it.


  1. John

    Open the windows (if the windows open) use fans, wear masks, as much distance as possible get the faculty and staff and eligible students vaccinated. The school in which I teach has been in session all year with, IIRC, three positive tests among students which resulted in quarantines for limited numbers. A few students have been attending via ZOOM because of particular family circumstances. Admittedly, this is a small school and we were able to spread out in ways that are most likely not available to larger schools.

    In any case, it has not been easy

  2. flora

    an aside of sorts:
    While the erstwhile prince Harry might find the US Constitution’s 1st Amendment unimportant, the 1st Amendment has allowed important public questioning of the original govt agency understanding of disease transmission, to the benefit of all, including the CDC.

  3. Edward

    I think schools should try to hold classes outside, if possible. Summer vacation could be moved to winter to facilitate this.

    1. Ed Miller

      Summer school in the northern states, but for the most part winter school in the southern states. Must consider climate in each area.

  4. The Rev Kev

    Could this be a CYA report? As far as medical advice is concerned, the quality of information coming from the CDC has been a bit of a shocker since day one. In spite of all the evidence pilling up about transmission by aerosol going back to that Washington State choir, they have refused to budge. Now that it is becoming more obvious that aerosols is the main factor here, this report turns up. Is it so that if there are any Congressional public hearings down the track, then the CDC can then say that when evidence did emerge of aerosol spread as the main cause of transmission, that they did put out a big report on May 21? Sort of how a bureaucrat would cover their a** by quietly putting a memo into the records saying that they opposed a project in case it goes belly up.

    1. Lambert Strether Post author

      > Could this be a CYA report?

      This is a family blog. That’s why I wrote:

      [T]he model has much to recommend it from the bureaucratic standpoint of always deploying a gluteal shield

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