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…).
- 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
Most outbreak reports (eg German meat p.) *do not include* information needed to investigate aerosol spread quantitatively:
- ventilation rate, easily measurable by:
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
CO2 (above ~400 ppm outdoors) indicates the amount of exhaled air (& virus) trapped in a space
Also per recent scientific results by @ukhadds, CO2 helps SARS-CoV stay infectious in air much longer
@united flight boarding, pretty terrible!
2/ This is the trip so far:
-Low outdoors
-Pretty high ~2000 in @RideRTD bus to airport
- ok ~800 at @DENAirport, except restroom ~1500. Not sure why restrooms at this airport are so often poorly ventilated
- Then boarding on @united, ventilation OFF, so huge increase till ON
3/ For details of the recent results on how and why CO2 makes SARS-CoV-2 stay infectious much longer in the air, see this recent thread by @ukhadds
1/ "After four years of fighting about it, @WHO has finally proclaimed that viruses, including the SARS-CoV-2 virus that causes COVID, can be spread through the air"
3/ "Words matter. When people heard that COVID might spread on surfaces, they wasted time wiping down groceries. People who misunderstood airborne spread needlessly wore masks on outdoor walks and veered off sidewalks to avoid their neighbors."
1/ New paper in @ScienceMagazine: "Mandating Indoor Air Quality for Public Buildings"
Explaining current status of indoor air quality standards (in short: bad or non-existent), the huge health benefits that would arise from them & proposing a path forward science.org/doi/10.1126/sc…
2/ "People living in urban & industrialized societies, which are expanding globally, spend more than 90% of time indoors, breathing indoor air (IA)."
"Most countries do NOT have legislated indoor air quality (IAQ) performance standards for public spaces"