The Science of Masks 3: Transmission, Aerosols or Droplets
The CDC doesn't even read their own references.
Welcome back to my review of the CDC guidance on masks plus my review of something actually worth your time, reruns of Dharma & Greg. Although much of what occurs on the show is intended to be absurd, the writers have nothing on the geniuses at the CDC.
In Part 2, we looked at CDC references about asymptomatic and presymptomatic transmission. This time we’re reviewing the two references that deal with the issue of bioaerosol transmission, and whether transmission is more likely via aerosols or droplets. This is so boring one of my hands fell asleep just typing that sentence.
Although a subtle point, this has some bearing on the attempt to justify mask mandates. Large droplets fall to the ground very quickly, while aerosols can remain suspended in the air for a long time. If only droplets are a concern, one could argue that simply placing some distance between people is a useful intervention. It certainly helps reduce exposure to halitosis.
Also, today is Episode 3 of Dharma & Greg: “Shower the People You Love With Love”. A wedding party is being planned, and Dharma and best friend Jane are going shopping for a shower while pretending to be deaf cockney hunchbacks. I’m not making that up - and it’s still less ludicrous than most of the pronouncements from the upper echelons of the CDC.
1. Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic (National Academies of Sciences, Engineering, and Medicine 20201)
The author of this letter (Fineberg) is responding to a request for advice (“rapid expert consultation”) on whether the existing literature supports the idea that SARS-CoV-2 can be spread via bioaerosols.
This letter responds to your question concerning the possibility that SARS-CoV-2 could be spread by conversation, in addition to sneeze/cough-induced droplets.
To accomplish this, the author has reviewed five different papers. The first of those papers states there was a widespread presence of viral RNA, however they do not know if the viral particles they found could cause infection.
A recent study of SARS-CoV-2 aerosolization at the University of Nebraska Medical Center showed widespread presence of viral RNA in isolation rooms where patients with SARS-CoV-2 were receiving care.
While this research indicates that viral particles can be spread via bioaerosols, the authors stated that finding infectious virus has proved elusive and experiments are ongoing to determine viral activity in the collected samples.
One study being reviewed, which ran from March 2013 through May 2016, looked at patients with known infections of human coronaviruses, influenza, and rhinovirus. This study found:
Among the samples collected without a face mask, we found that the majority of participants with influenza virus and coronavirus infection did not shed detectable virus in respiratory droplets or aerosols, whereas for rhinovirus we detected virus in aerosols in 19 of 34 (56%) participants (compared to 4 of 10 (40%) for coronavirus and 8 of 23 (35%) for influenza). For those who did shed virus in respiratory droplets and aerosols, viral load in both tended to be low.
These findings suggest that surgical face masks could reduce the transmission of human coronavirus and influenza infections if worn by infected individuals capable of transmitting the infection.
So this particular study provides mechanistic evidence for the use of masks, but is not a study of the transmission of these viruses.
All of the studies cited by Fineberg generally support the conclusion that the virus could be found in bioaerosols. As described earlier, this author also points out that the relative importance of the different possible transmission paths is not known (as mentioned earlier).
However, for no respiratory virus is the exact proportion of infections due to air droplet, aerosol, or fomite transmission fully established, and many individual factors and situations may contribute to the importance of each route of transmission.
And finally, as stated above, the author concludes:
While the current SARS-CoV-2 specific research is limited, the results of available studies are consistent with aerosolization of virus from normal breathing.
[Hearing Dharma has plans for a shower, Greg’s mother Kitty has rushed to the apartment only to discover that they were talking about a shower shower, not a bridal shower. The apartment has no shower (with just a tub, Greg can’t wash himself in the correct order) so she and Jane are going to install one. You go, Dharma.
Kitty: “Why do you people live like this?”
Dharma: “Because you can’t camp on the beach anymore”]
2. Lack of COVID-19 transmission on an international flight (Schwartz et al. 20202)
This is a short letter documenting the testing on passengers from an international flight from Wuhan, China to Guangzhou, China and then to Toronto, Canada. The index patient and the wife of the index patient tested positive for SARS-CoV-2 and close contacts with those patients were monitored by public health officials in Canada for 14 days after the arrival of the flight.
One close contact developed a cough, and five other people who were not close contacts also reported symptoms. All six of these people tested negative for SARS-CoV-2. Based on the fact that no one on the flight developed SARS-CoV-2, the authors conclude that droplet rather than aerosol transmission is the likely route of transmission.
... the lack of secondary cases after prolonged air travel exposure supports droplet transmission, not airborne, as the likely route of spread of the COVID-19.
So no actual transmission of the disease is observed, and the authors conclude something about the mode of transmission. Given that there were 350 passengers on the plane and this flight takes around 18 hours the lack of secondary cases seems like positive news.
The authors don’t give much explanation for how no one getting sick allows them to conclude the mechanism of transmission. I plan on making a fortune selling them elephant repellent. (“Look, I sprayed the repellent and there have been literally zero cases of elephant stampedes in this hospital. Clearly my product works.”)
[Update from future me: This also happens to be my favorite dumb reference so I wrote a whole sarcastic post about it, titled My favorite dumb reference.]
Summary of the science part
Someone at the CDC asked someone at the National Academy of Sciences for a quick review of the literature to determine if maybe the virus could be spread during normal conversation. That totally doesn’t at all read like maybe someone had an agenda involving slapping masks on regular folks doing regular things like talking.
Also, a hundreds of people were sealed in a metal tube for about 18 hours with a pre-symptomatic patient and none of them got the ‘Rona. I found this surprising, given that based on CNN reporting I thought it spread like the zombie virus in Dawn of the Dead.
Summary of the D&G part
Greg and his friend Pete also bought a shower and had time to install it while Dharma and the ladies were out. See, they both had the same idea - it’s Karma. The show doesn’t really explain what Karma does with the other shower.
Next up is Part 4: “And Then There’s Some Lab Experiments” along with Episode 4: “And Then There’s the Wedding”.
National Academies of Sciences, Engineering, and Medicine. 2020. Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic (April 1, 2020). Washington, DC: The National Academies Press.
https://doi.org/10.17226/25769
Schwartz KL, Murti M, Finkelstein M, et al. Lack of COVID-19 transmission on an international flight. CMAJ. 2020;192(15):E410. doi:10.1503/cmaj.75015https://pubmed.ncbi.nlm.nih.gov/32392504/