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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: https://twitter.com/ShellyMBoulder/status/1306352867213631488
52 out of 60 infected after 2.5 hr practice
Our Skagit Choir Paper has been accepted after peer review, available at: https://twitter.com/ShellyMBoulder/status/1306352867213631488
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
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.)
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”
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%.
“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” https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html).
Fomites general I: everyone agrees not very efficient, e.g. CDC: “not thought to be the main way the virus spreads” https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html).
7/ Fomites general II:
- UK hand washing study reduced trans. 16%: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/897598/S0574_NERVTAG-EMG_paper_-_hand_hygiene_010720_Redacted.pdf
- Enveloped viruses don’t live long on hands: https://pubmed.ncbi.nlm.nih.gov/6282993/
- UK hand washing study reduced trans. 16%: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/897598/S0574_NERVTAG-EMG_paper_-_hand_hygiene_010720_Redacted.pdf
- Enveloped viruses don’t live long on hands: https://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
- 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
- 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**
- 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
- 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
- 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
- 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
- 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
"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” ( https://www.who.int/publications/i/item/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations)
Let’s hope @WHO takes note of this & other outbreaks reports w/ obvious aerosol transmission in next brief
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 http://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
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
- 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: http://tinyurl.com/faqs-aerosol
- Any feedback is welcome as always
- Choir practices risky during COVID-19
- Mitigation: http://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: https://twitter.com/jljcolorado/status/1289360404271861760
- volume of the room
Pls include in future reports
Most outbreak reports (eg German meat p.) *do not include* information needed to investigate aerosol spread quantitatively:
- ventilation rate, easily measurable by: https://twitter.com/jljcolorado/status/1289360404271861760
- 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
We thank choir & *especially* spokesperson Carolynn Comstock, & Skagit County Public Health Dept. for extensive support of our investigation
Thread roller version; https://threadreaderapp.com/thread/1306450428867964930.html
