1/ Another thread on the whole airborne thing. But this one might be a bit different, and I hope it serves as a bridge to critics. I want outline how I became so invested in the problem myself, and address what I hope are the last few stumbling blocks in Canada.
2/ Anyone who has been paying attention is familiar with many of the arguments both for and against airborne transmission of COVID-19. Those arguing for airborne transmission, at least initially, tended to come from applied science backgrounds and were viewed as outsiders.
3/ Why would applied scientists focus on this problem? I think many of these scientists found themselves deeply unsatisfied with how transmission was being described. At the start of this pandemic, I myself questioned why we were only concerned with large drops and not aerosols.
4/ Everything I knew about aerosols ran counter to what I was seeing described in messaging to the public. My research group agreed, and we put out feelers (mostly unanswered) towards institutions to outline our concerns.
6/ When the people who study aerosols for a living saw the way aerosols were initially being described, alarm bells set off. A good engineer/applied scientist is never satisfied with the explanation for something if it is not grounded by a reasonable physical explanation ...
7/ that stands to scrutiny. This mindset is a result of our training, built upon decades of real-world experience with topics like risk-reduction, design, and rigorous study of the physical sciences.
8/ Bioaerosols inhabit this weird grey area between a supremely complex biological system (the human body) that essentially necessitates the use of RCTs to determine anything useful, and simpler systems that can be really well characterized using the basic sciences.
9/ From my perspective, the basic sciences provide us with an overwhelming amount of ammunition in describing how bioaerosols work, and give us ample hints as to how to address them.
10/ For me, the description that droplets fall to the ground immediately, and that anything bigger than 5 micrometers in diameter was a droplet, was when I knew I had some knowledge that was applicable. This belief was reaffirmed with the NASEM workshop ( https://www.nationalacademies.org/event/08-26-2020/airborne-transmission-of-sars-cov-2-a-virtual-workshop).
11/ We have since seen more and more work that has pointed towards the cracks in the traditional IPAC framework that divides transmission into contact, droplet, and airborne.
12/ This framework has traditionally served us well, but at a point where it was stressed beyond any measure of previous experience with a respiratory disease its inherent limitations became magnified. (Note: that's OK, science works by finding out what works and what doesn't).
13/ The biggest problem is the lack of consideration of inhalation as a route of exposure over short distances. What would seem to be an obvious conclusion - that inhalation over short distances is important - evades proper consideration in the contact droplet airborne framework.
15/ There have been many arguments put forward to justify COVID-19 not being airborne. These have been addressed ( https://www.journalofhospitalinfection.com/article/S0195-6701(21)00007-4/fulltext, https://aricjournal.biomedcentral.com/articles/10.1186/s13756-020-00868-6) such that there is no question that transmission via inhalation of infectious aerosol is an important consideration.
16/ People may disagree on the degree to which COVID-19 is airborne, but an absolute denial of inhalation as an important route of transmission is not really possible.
17/ The arguments supporting airborne transmission of COVID-19 (via inhalation at short or long range) are well supported by physical scientific reasoning. Every process, from the generation of bioaerosol in different regions of the HRT, ...
18/ the statistical distribution of virus among bioaerosol, the dilution of bioaerosol with distance from the source, the roles of masks/respirators and ventilation, etc., is sensible and holds up under scrutiny.
19/ So what are some of the remaining roadblocks to accepting COVID-19 as an airborne disease? I believe the major one is the idea that we just aren’t seeing as many outbreaks among HCW as we would expect for a truly airborne disease.
20/ HCW, in Canada at least, have typically used contact and droplet precautions. If COVID-19 was truly airborne, wouldn’t we expect to see more infections among HCW?
22/ One thing to remember is that not all bioaerosol is dangerous; statistically speaking, most of the individual droplets contained in a bioaerosol will NOT contain viable virus. So, when we say that coughing produces 1,000’s to 100,000’s of aerosols per minute, ...
24/ If you’re unlucky and unmasked, you might inhale a lot of these particularly dangerous particles very quickly if you’re standing near the emitter, enough for a particular virus to find a vulnerable pathway and lead to an infection.
25/ You may also be sprayed in the face during a sneeze or cough, so there's certainly potential for drops to hit your mucous membranes if you are really unlucky. Statistically, though, inhalation is vastly more probable.
26/ But back to the ?, it’s not entirely clear how many infections are occurring in HCW, nor is the extend of nosocomial outbreaks in Canada fully known (reports have yet to be released to the public for several outbreaks in Alberta).
27/ In cases where outbreaks occur at hospitals, publicly stated causes tend to be breaches in protocols rather than a failure of PPE. Given that inhalation has been a neglected route of transmission, this isn’t a surprising conclusion.
28/ However, there are a few things that may work in our favour to reduce the likelihood of airborne transmission via inhalation among HCW. a) Hospitals in Canada (more modern ones) tend to be built with airborne transmission in mind.
29/ If a modern hospital is up to code (CSA Z317.2:19, and legacy versions) and systems are working properly, it will have good ventilation that will help to reduce the buildup of infectious aerosol in a space.
30/ b) Patients in the hospital (in or out of the ICU) may be past their peak period of infectiousness, and many may not be shedding enough virus to cause transmission after their disease is severe enough to require hospitalization.
31/ Viral shedding peaks just prior to symptom onset ( https://www.acpjournals.org/doi/10.7326/M20-5008), with viral loads in the respiratory tract tailing off towards negligible levels about 2 weeks after symptom onset. It takes a little while for patients to require hospitalization
33/ This would be of tremendous benefit in avoiding regular nosocomial transmission. c) COVID-19 is a highly overdispersed disease, with 10 to 20% of people causing ~80% of transmissions.
34/ There might be a link between these 10% of super-spreaders with the bioaerosol super-emitters that have been observed in the literature ( https://www.nature.com/articles/s41598-019-38808-z), but this remains to be studied. Many patients will belong to the 90% that cause fewer secondary cases.
35/ d)Surgical masks used with droplet precautions provide some *limited* protection against airborne hazards. Oberg & Brosseau found that surgical masks gave fit factors of between 2 and 10 https://pubmed.ncbi.nlm.nih.gov/18455048/ , corresponding to 50% to 90% filtration of ambient aerosol.
36/ Coupled with factors I – III above, this layer of protection can certainly reduce the risk of transmission via inhalation from occurring.
37/ d, caveat) That being said, we have much better tools that provide a much stronger layer of protection against inhalation of infectious bioaerosol, (e.g. respirators like the N95 and better).
38/ Why rely on a substandard (and many would say completely inappropriate) tool when we have better options available, particularly now that supply of respirators is less of an issue than at the start of the pandemic?
40/ https://www.acpjournals.org/doi/10.7326/M20-7567). Given the neglect of inhalation as a plausible explanation, I suspect it’s more common in Canadian care facilities than currently thought.
41/ On the flip side of this, we have community transmission, where none of the above factors really work in our favour. Inconsistent mask use, peak infectivity overlapping with symptom onset, poor ventilation in many public spaces, etc. etc.
42/ all conspire to create conditions that favour airborne transmission. If COVID-19 is an opportunistic airborne disease, we’re giving it lots of opportunity.
43/ So where does this leave us? Well, we’re in the lead up to wave 3, and we’re racing to both vaccinate and reopen as soon as possible. Pessimistically, I don’t see us changing our messaging in Canada to reflect airborne transmission, but I’ll continue to advocate we should.
44/ Optimistically, other jurisdictions have embraced aerosols and the roles of ventilation/better masks (like Germany, Austria, Japan, New Zealand, Australia, etc), and it will be interesting to see how they fare over the next few months as vaccination rates increase.
45, fin/ I’ll be watching with envy and a hope that I was able to provide some Canadians with information they can use to avoid the most dangerous situations.
Also, HRT = human respiratory tract. Too many acronyms sometimes...
My lovely partner pointed out that I have a glaringly obvious typo in the very first tweet of this thread, and now that's all I can see. I should have had her read this *before* I posted it 😐.
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