During Friday's debate with @kprather88 and @DFisman, Dr. Conly raised a fairly common counter-point for why he thinks COVID is NOT Airborne: the relatively low Secondary Attack Rate (SAR) and Reproduction Rate (R0).
Let's unpack this flawed argument in a 🧵
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Dr. Conly states the SAR is 3 to 10%, with a household mean of 18%.
He then correctly points out that there is significant heterogeneity within the data, which means some people transmit to no contacts, while others transmit to everyone.
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After admitting to the heterogeneity, he cautions we "have to be careful not to draw conclusions" from these superspreaders... eg. Chalet (73% SAR), Choir (53%), and Diamond Princess (58.9%). He suggests there were "extenuating circumstances", and seems to dismiss them. 🧐
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First note of caution... there are several recent studies that suggest a higher SAR than what Dr. Conly has referenced. Here's one from the CDC, which finds a SAR of 53%.
The other argument made by Dr. Conly, is that the R0 is "consistently in the range of droplet/contact viruses".
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Is it though?
1) Estimates for R0 of measles range from 5 to 20+ (see thread) 2) Other estimates for R0 of SARS2 are as high as 11.. 3) Why wasn't TB plotted? Its an Airborne virus and the R0 is ~3 🧐
Here's what the plot looks like when you include the variable R0 of measles, the variable R0 of SARS2 and the R0 of TB.
Not so clear cut now, is it? 🧐
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As an aside, R0 of an Airborne virus makes little sense to me. The number of people infected in a scenario, will be heavily dependent on ventilation. It would be better to have an R0 normalized by ACH, or some other measure of fresh air. Then you can compare apples to apples
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So why the vast variability in R0 and SAR for SARS-CoV-2 across these studies?
This is indicative of the overdispersed nature of this virus, which @DFisman explains here:
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Dr. Fisman explains this Overdispersion via 2 Epiphanies:
1) Variability in Aerosol production, depending on vocalizing activity and variability in Aerosol build-up depending on environmental conditions
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2) Variability in Viral Load from person to person and by day of infection
Put all of these together (variability in aerosol generation, ventilation and viral load), and it becomes obvious why this virus is so overdispersed.
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So the heterogeneity in the SAR/R0 is actually extremely important.
The overdispersion means the superspreader events are driving the Pandemic.
This is elaborated on further in this Nature article.
As Dr. Conly reminded us on Friday, when considering Airborne Transmission, "it's very important to take into consideration the very very complex environment".... "direction of airflow, the number of air changes, temperature, relative humidity... its veeery very complex"
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This is actually NOT "complex" for Aerosol scientists, mechanical engineers, building scientists, and other experts in fluid mechanics. Due to the Pandemic, these experts have diverted their brainpower to this problem. See summary of studies below.
Let's unpack Dr. Conly on the Precautionary Principle.
Q: Why weren't HCW provided N95s, given the advanced warning that SARS2 might be Airborne?
Conly: "I need to see a much higher level of evidence..."
(🤔 That's not how the Precautionary Principle works)
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He then makes a pretty bold claim:
"certainly from our experience in Alberta, in Phase 1" the HCWs wore "gowns, gloves, medical masks"... and "not a single HCW transmission"... despite "highly infectious, hot viral patients"
*Phase 1* is defined here: MARCH-APRIL 2020
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So why the "Phase 1" qualifier? Why is he using evidence from a 1 to 2 month period that was over a year ago, and extrapolating out to "8 months of continuous contact time", rather than just telling us what happened during the past year?
Re. "Is this really a debate?". Watch this video for yourself...
While this is from July 2020... so far there has been no public movement from Dr. Conly or the WHO IPAC committee on this issue. If anything they have dug-in their heels further.
Just want to clarify that it's likely the Mother was the Index case. Child tested negative at the hospital. Mother was asymptomatic but developed symptoms 2 days after discharge, tested positive on day 4. Child tested positive on day 6.
Another key quote:
"As some of the report's authors were taking part in the medical round and got infected, we can declare there was full compliance with PPE guidelines."