1/ The Skagit Choir outbreak was dominated by aerosol transmission

52 out of 60 infected after 2.5 hr practice

Our Skagit Choir Paper has been accepted after peer review, available at:
2/ Not isolated event. Other choir superspreading events in: Netherlands, Austria, Canada, Germany, England, South Korea, Spain, and France

We started investigating this event shortly after the LA times report by @RichReadReports
3/ Submitted in June. Aerosol transmission seemed pretty obvious, didn’t dwell on demonstrating it

We were surprised to find skepticism that aerosols led to the outbreak (including @WHO), so in revised version we added extensive description of event (in paper and Supp. Info.)
4/ In this thread we review some of the information why we think aerosols dominated

County Public Health & choir members concur that choir practice was were infection happened

Choir spokesperson: “It is not a highly social group. It is pretty seriously about the music”
5/ Known index case

“No one had contact with the index case during that week” [prior to the rehearsal]

Some of the time split into 2 groups in rehearsal, attack rate in room where index case stayed as expected

Prob. of 2nd index case: 0.3%.
6/ 3 modes of transmission: fomites (surfaces), ballistic droplets, and aerosols

Fomites general I: everyone agrees not very efficient, e.g. CDC: “not thought to be the main way the virus spreads” cdc.gov/coronavirus/20…).
7/ Fomites general II:

- UK hand washing study reduced trans. 16%: assets.publishing.service.gov.uk/government/upl…

- Enveloped viruses don’t live long on hands: pubmed.ncbi.nlm.nih.gov/6282993/
8/ Fomites in choir I:

- aware of COVID-19, no one reported any direct personal contact

- entrance door propped open, used sanitizer

- index case did not handle chairs, did not have snacks

- ½ had snacks. Many who did not have any snacks got sick
9/ Fomites in choir II:

- index went to specific bathroom that only ~3 other people used

- Many who did not use any bathroom got sick

CONCLUSION for fomites: extremely unlikely to explain more than very few cases
10/ Second mode of transmission: Ballistic droplets I:

- Not too close: 61 ppl where normally there would be 120

- Infections everywhere in the room, w/o clear pattern. Many cases behind index. **Infections up to 44 ft (13.5 m) from (& behind) index case**
11/ Ballistic droplets II:

- index case did not have anyone within the 3 m (10 ft) likely landing area for droplets emitted while singing. Droplets don’t travel backwards

- Many ppl (including index) came in just in time to start and left right at the end
12/ Ballistic droplets III:

- Only 1 x 10 min. break. Rapid transitions otherwise, not much talking

- Index case talking to others was minimal during break (& entire event). Others talked to 2-3 ppl on average during 10 min. break
13/ Ballistic droplets IV:

- CDC: need 15 min. Of close contact to get infected.

CONCLUSION for droplets: seems impossible to infect 52 people by impacting droplets on their eyes, nostrils, or mouth in this event
14/ 3rd pathway: aerosols (float in air, infect by inhalation)

- aerosol emission much higher when singing

- air was well mixed within the room because of convection due to heat emitted by ppl

- Ventilation was low

- CONCLUSION: aerosols can easily explain the outbreak
15/ Summary of transmission in Skagit Choir rehearsal:

"Per Occam’s razor, explanation most probable: inhalation of infectious respiratory aerosol from “shared air” was leading mode of transmission"

Unfortunate combo: high occupancy & duration, loud vocalization, low vent
16/ @WHO said on their last brief “detailed investigations of these clusters suggest that droplet & fomite could also explain these clusters” (who.int/publications/i…)

Let’s hope @WHO takes note of this & other outbreaks reports w/ obvious aerosol transmission in next brief
17/ We then model event w/ box-Wells-Riley model (similar to tinyurl.com/covid-estimator), conclude index case was exhaling ~1000 infect. doses / hr

Quite high: 6-16x higher restaurant or bus

Quant. consistent w/ known much higher aerosols singing loudly vs talking intermittently
18/ What could have helped limit outbreak?

- Rehearsal outdoors would have been most effective

- Masks, ventilation, portable filters, shorter duration: all help, none silver bullet. All together: 52 cases → 5 cases
19/ CONCLUSION: Skagit Choir superspreading event (52 cases) was dominated by aerosols

- Choir practices risky during COVID-19

- Mitigation: tinyurl.com/faqs-aerosol

- Any feedback is welcome as always
20/ For ppl investigating outbreaks:

Most outbreak reports (eg German meat p.) *do not include* information needed to investigate aerosol spread quantitatively:
- ventilation rate, easily measurable by:
- volume of the room

Pls include in future reports
21/ Authors: @shellyMBoulder, W. Nazaroff, A. Boerstra, G. Buonanno, S. Dancer, J. Kurnitski, @linseymarr, L. Morawska, @CathNoakes & me

We thank choir & *especially* spokesperson Carolynn Comstock, & Skagit County Public Health Dept. for extensive support of our investigation
Thread roller version;

And we need to explicitly thank @RichReadReports, whose great article alerted us to the outbreak, and who bent over backwards to help us in any way he possibly could


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More from @jljcolorado

29 Sep
1/ What does "aerosol" or "airborne" transmission suggest to people?

I am impressed by the accuracy of the responses. Indeed the **most important implication is that transmission occurs in shared room air**, and protecting against that. Image
2/ Transmission in close proximity is ALSO dominated by aerosols (sciencedirect.com/science/articl…).

But since social distance and masks work well for that, the practical implications of accepting aerosol transmission are much smaller for that.

And indeed many fewer ppl think of that
3/ Trans. beyond shared room ("long range") possible, but much less likely.

Won't be clear till there is low prevalence.

E.g. long range trans. for smallpox could only be proven for an outbreak in complete absence of community transmission: academic.oup.com/aje/article-ab…
Read 5 tweets
29 Sep
Rueda de prensa de ayer (aerosoles y COVID-19) en YouTube:

Minuto 59:00 hablo claro del error tremendo de las 5 um, y que si alguien todavía dice eso (como la OMS), NO se puede confiar en nada de lo que dicen sobre aerosoles
2/ Y he puesto la otra rueda de prensa (3-Sep) y dos presentaciones de introducción a la transmisión por aerosoles y al estimador de transmision (tinyurl.com/coavid-estimat…) en espanol e ingles
3/ Gracias por la sugerencia de poner los videos que voy haciendo en YouTube para que sean más fáciles de encontrar.

Mi hijo de 6 años (que ve YouTube mucho) estaba muy impresionado, "Papa es un Youtuber!" hasta que vio 1 minuto de un video y dijo "muy aburridísimo!"
Read 6 tweets
26 Sep
Esto que dice @FiltrosHepa son verdades a medias o mentiras
- Cuantos virus quitan del aire depende de su capacidad comparado con la habitación. He donado para la escuela de mi hijo unos que limpian el aire de la clase cada 12 minutos
- Simplemente colocarlos en lugar central
"Un filtro HEPA solo va a aportar una falsa sensación de seguridad"

Esto es totalmente falso. Si un filtro es muy pequeño comparado con la habitación, tal vez. Pero si es de tamaño suficiente, esto es totalmente falso
"El aula más segura es el aula en casa, con recursos online."

Esto es correcto, por supuesto. Mejor por internet.

Pero si obligan a ir a un sitio cerrado, al menos los filtros HEPA reducen el riesgo. Simplemente no ponerlos en una esquina sino el em medio
Read 8 tweets
24 Sep
1/ Aerosol scientists don't agree. What kind of consensus is that?

Does the discipline who studies transport of particles through the air, NOT count for a "consensus" of which particles travel through the air??
2/ this "consensus' is incorrect.

SARS-CoV-2 transmits better to people nearby because aerosols are most concentrated there. And then it can transmit in a room, if we "help it" w/ low ventilation, long time etc.
3/ If it can infect in a room, it MUST infect much more easily in close proximity, where it is ~50-100 times more concentrated than in room air.

Just like smoke (an aerosol), which is far more concentrated in close proximity than at the room level.
Read 5 tweets
24 Sep
A5(1) It is clear that ventilation reduces transmission
- Ballistic droplets & close proximity aerosols not affected by vent.
- Only shared-room-air aerosol transmission impacted by ventilation. This is a major pathway of transmission

#LeapsChat @leapsmag
A5(2) "ventilation" here = remove virus from the air
- A fan that moves air around is not ventilation

- 2 things:
(a) expel indoor air outdoors, bring clean air indoors (open windows, or through mechanical HVAC systems)

(Details: sciencedirect.com/science/articl…)

#LeapsChat @leapsmag
A5(3) Second thing to do:

(b) Filtration: either through HVAC systems (MERV 13 if possible), or through portable HEPA filters, or through cheaper fan-filters.

Extensive details: tinyurl.com/FAQ-aerosols

#LeapsChat @leapsmag Image
Read 5 tweets
24 Sep
A2.(1) Major historical error in 1910 became paradigm:
+ correct observation (ease of infection in close proximity) was misinterpreted as being due to ballistic droplets
+ Stated that aerosol infection was nearly impossible


#LeapsChat @leapsmag
A2(2) Unfortunately paradigm has reigned till today

For COVID-19, and other diseases, droplets are accepted w/o evidence. But aerosol evidence is NEVER enough

As stated by Chapin in 1910: aerosols very unlikely, the burden of proof is on their proponents

#LeapsChat @leapsmag
A2(3) The unquestioning proclamation of droplets as the main mode of transmission by @WHO and @CDCgov
is especially troubling, given that they have never been demonstrated to transmit any disease in the history of medicine

#LeapsChat @leapsmag
Read 5 tweets

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