Thread by @DrSimonAshworth

@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM Respiratory viruses are transmitted by particles along a spectrum of sizes. The starting assumption was that the main mode of transition was large particles deposited on surfaces (fomites) which we touched and put in our mouths…
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM This partly arose because it was assumed that SARS1 was triggered by people pressing the button in a hotel lift and then going off around the world…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM And this probably does happen… so wash your hands… the next and obvious mode are large droplets emitted when someone coughs in your face… pretty obvious but contentious it’s an inescapable conclusion that masks would have a role and that was controversial in 2020
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM But coughing emits a lot of big droplets which fall quickly to the ground, though they can travel 6-8m they are in the air for barely 1 second…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM But the problem was there were instances in SARS1 and in early SARS-CoV2 where direct or fomite spread could not be the mechanism. For example where people in remote parts of a building caught the virus, or when there was a lag between emission and infection
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM Also we noticed that SARS-CoV2 was particularly prone to superspreader events where on infected person cause dozens or hundreds of new cases - this happened in meat packing plants
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM So what wasn’t accepted was that breathing, speaking, singing and talking emitted smaller particles, that these contained virus, and that these particles became suspended in the air and floated around propelled by air currents and caused infections
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM And even if this was accepted, it wasn’t accepted that this was an important, still less potentially the main mode of transmission

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM We come onto masks which were also controversial. The fomite (surface to hand to mouth) model had led to the worry that masks would be harmful because people might ingest particles on their masks - hence resistance to masks…
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM The problem is you could not square this with the fact Asian countries used a lot of masks and controlled SARS very fast… but of course they did a lot of other things…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM Also “fabric masks” don’t have a tight enough weave to stop the smallest particles getting through, but they could stop larger particles if someone coughed at you… and also of course they reduce the distance particle travel

So reducing infection still makes sense

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM And there just wasn’t much virus found when we sampled mass transit areas, and the density of viral particles in the air in hospitals was very low

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM But what was also important was that patients who were dying were getting pneumonia… so how was the virus getting to the lung… it was believed that this was because it replicated in nasal or oral tissues and dripped down…
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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM This belief was based on the fact big particles get deposited in the mouth throat and nose and don’t get further directly - only the much smaller airborne particles could get further down

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM And here we get to the nub. But what if it was just inhaled directly? Why do you need a middle man?

After all the sick patients were dying of pneumonia…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM Further the lungs are 30-50m2 and we breathe 3-200 L/min. So what if we hoover in giant quantities of air which has a few tiny suspended viral particles onto a large surface and they directly infect the lungs?

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM But to come onto the question in healthcare we have been obsessed by “aerosol generating procedures” AGPs. This was reinforced by the fact that a lot of anaesthetists died after putting breathing tubes into SARS1 and SARS-CoV2 patients…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM And there were very legitimate fears that procedures like CPAP and noninvasive ventilation generated aerosol (tiny particles). But in fact medical AGPs mainly generate larger droplets…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM The intubation risk is more likely to be a mix of passive expiration of tiny particles and being very close to the patient so droplets go straight in your face - it is high risk

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM But work from Bristol has shown that procedures like CPAP may if anything reduce the generation of particles that generate suspended viral particles

@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM Any additional risks are arise if the mask mask gets removed and you are nearby and get a face full of larger droplets, just as if they had coughed at you, in which case standard face masks probably provide good protection

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM This isn’t to say there is no aerosol… it is likely that anyone with early #COVID19 / #SARSCoV2 emits tiny aerosol particles which become suspended in the air simply through breathing…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM What protects us from airborne viral particles floating in the air?

1. Good ventilation dilutes and removes these particles

2. High filtration masks like N95/FFP2, N99/FFP2 will also provide excellent protection

3. Vaccination

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM If we have made one mistake it is to oversimplify.

SARSCoV2 can spread via fomites, it can spread via droplets, and it can spread via airborne particles. But my suspicion is the latter is the most important…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM It is possible to reduce all of these risks, with better ventilation, good cleaning, hand washing, surgical face masks in non-enclosed areas and higher performance masks in poorly ventilated areas… and we may need to combine these…

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM One added challenge is ventilation is expensive and environmentally unfriendly, but especially if there is a high density of people, window opening isn’t enough

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@LoyalStingray @kprather88 @ChrisCarrollMD @First10EM But we must get away from half baked measures which just give the illusion of doing things, and we must get away from thinking we have beaten this virus without objective evidence confirming really effective suppression

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