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, 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 . 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) “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:
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. is an important COVID-19 prevention strategy in schools as part of a multicomponent approach.
“ 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!
 This was the post that Google suppressed, or at least had an extended hiccup while adding it to their search index.
 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.