WHO says 'evidence emerging' airborne COVID-19 spread may be a threat indoors

“Evidence is emerging” that the WHO is not really up to the task.


Short interjection: airflow in airplanes might actually be less likely to spread aerosols than in most other circumstances where indoors ventilation is present.


I’m not super confident in any of these people. To put it in Dunning-Kruger terms, it has been hugely obvious since the very beginning that Coronavirus is airborne and asymptomatic people are very contagious. But the experts are still wringing their hands with uncertainty about that.

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Thanks for breaking this down and explaining it. I fear many people will have knee jerk reaction’s upon hearing this news and will confuse aeresol transmission with droplet transmission and not listen any further thinking they have already heard all this. And of course the steps that the experts are recommending: upgrading/maintaining filtration systems and letting more fresh air in, will go unheeded because of this and we’re all fucked.


WHO says ‘evidence emerging’ airborne COVID-19 spread may be a threat indoors?

Daaaaamn right.

Every furnace/AC system has a filter. And while many people don’t change them often enough, performance generally increases with time. The reason you change them is because they obstruct airflow too much when they get plugged. Some of them are actually rated for removing viruses, others are not, but even standard filters will probably beat out your cloth mask. In any case it is a cheap and easy upgrade. It’s not optimal as most furnace intake ducts are sized for a less restrictive filter so airflow will be reduced especially as the filter ages and gets more plugged, but at least at the moment it makes sense for pretty much anyone operating a public facility to replace the air filters with high MERV rated ones, ideally ones specifically rated to remove viruses and other very small particles.

An interesting compromise would be washable electrostatic filters. They obstruct airflow less than a pleated paper filter and are actually poorer at removing large particles like dust and pollen but are more effective at reducing sub-micron particles like aerosolized viruses.

As far as I can tell transmission through HVAC ductwork is not currently considered a major threat. The concerns I have seen about the role of ventilation in coronavirus transmission are more along the lines of three tables in a restaurant in the line of a AC vent. If someone at the first table is sick, the virus can be carried downstream much farther than 6’ and infect the people at the next tables. But a filter upgrade is so cheap and easy it makes sense especially given how much we still don’t know.

On the other hand, if anyone has references about transmission via HVAC ducts that I don’t know about, please post it.

Long term, as a chronic allergy sufferer and someone who would like to avoid getting preventable infections I would like to see furnaces designed with “oversized” filter slots so that we can use more effective filters without compromising airflow.

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Back when a 737 would stay airborne when it ran into problems.

Well, no. Experts recognized that almost immediately, but there was and is still a lot of uncertainty in exactly the mechanisms that are most important. (aerosol vs. droplet).

That said, the CDC and WHOs communication especially on “asymptomatic” transmission was a giant shitshow. Not because of a lack of understanding or anything, strait up terrible communication. The distinction they made between “asymptomatic” and “presymptomatic” transmission is largely pointless in terms of public policy and irrelevant for public communication. Asymptomatic transmission has a clear obvious meaning to most people who hear it, including from what I have heard many infections disease specialists: people transmitting the disease when they aren’t showing symptoms. Whether they eventually show symptoms is largely irrelevant. This is particularly silly for COVID when its known that many cases have mild symptoms which might only be recognized as symptoms retrospectively after a positive test. But they stuck to this pointless nomenclature and kept saying things like “asymptomatic transmission is rare, [several irrelevant sentences] pre-symptomatic transmission is common.”

All of this fed junk-science coming from people other than public health experts that was saying things like “there are 99% asymptomatic cases and we are nearing heard immunity” back in april. So it was a giant fuck-up, but not because the experts didn’t have a good idea of what was going on or a fair estimate of their uncertainties. It was a failure of communication from public health officials compounded by a complete absence of leadership to turn the justifiably uncertain knowledge from the public health experts into concrete “best effort” recommendations.

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Well, obviously there is confusion over terms. But the key phrase in “under certain circumstances” the virus may act like an airborne illness. Chicken pox it is not, but in an enclosed area with inadequate turnover of air volume, it can be quite a problem. Since we are rarely in total control of those variables, avoiding indoor crowds remains the wise choice.


But that isn’t what the evidence shows and this is one of those cases where the technical terminology readily confuses people. The current best evidence is that truly asymptomatic people aren’t a major transmission risk, but presymptomatic and mildly symptomatic cases are a large risk. There’s a similar distinction in the airborne versus droplet issue. Sadly I don’t think the airborne droplet distinction matters as much for political reasons, because I can’t see many political figures pushing for full airborne precautions.

Of course, these groups can only be sorted in retrospect. Also, the definition of “mildly symptomatic” is pretty suspect, as it can be tweaked to make the outcome whatever the researcher is looking for. I have advised my staff to take the same universal approach we did with HIV. You can’t tell if they (or you) are sick or contagious, so treat everyone as if they are.


@ejeffrey and @moortaktheundea as of right now, I’ve been aware they were drawing a distinction between “asymptomatic” and “presymptomatic” for the exactly three minutes since I read your posts. That seems like a pretty good illustration of your points about poor communication to the general public.

Since we can’t tell the future, presymtomatic vs. asymptomatic doesn’t really matter for most purposes. It is just confusing terminology that was repeatedly misapplied.

Mildly symptomatic is also tricky. If you read some of the statements from the WHO, they say things like “when we go back and check people classified as asymptomatic we find out they did indeed have symptoms”. Some of this is undoubtedly people who were pre-symptomatic, but it also appears that in some cases they are retroactively recognizing mild symptoms that the patient didn’t associate with COVID until they had a positive test result. You can cough, feel tired or otherwise sick because of allergies, cold, exercise, stress, or fatigue. I can tell you first hand that if you have a very active and social four year old only child stuck at home, you would need have to have pretty severe symptoms before thinking it was any sort of illness.


People aren’t even wearing masks, so… [throws up hands in apocalyptic despair]


Glad to help. That has been a really frustrating part of this.

I think the WHO is a little late to the party, here.

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In any higher risk situation we pretty much need to assume everyone is high risk. The razor distinctions are mostly only useful in pruning the search tree when doing contact tracing and some research.

Almost no one knew that distinction was being used, which is why the WHO had to walk back their press release a while ago, because almost everyone misunderstood it.

It doesn’t matter, except in things like contact tracing and a few other highly specialized niches. The catch is that WHO statements are read by both the specialist audience and a lay audience these days. Similar terms with really nuanced meanings work when the audience is largely trained specialists, but they really need some type of media team given that everyone is reading their releases now with high stakes.


I get that it in specialized situations the distinction does matter. However, in my limited understanding from outside the field, that jargon is not even consistently used that way in the infectious disease community. I have heard multiple but anecdotal claims by people working on fighting other infections diseases that they call “asymptomatic transmission” transmission by people not showing symptoms at the time, and then further divide that with other terminology that makes clear the difference between people who eventually show symptoms vs. those who don’t. This makes more sense to me, and if the community itself doesn’t have a single standard meaning for those terms, using the less confusing versions seems better to me.

Still, I am not in the field, and I understand how jargon works and why they use it. So I understand why scientific papers may describe things in a way that isn’t the best for public consumption. Some of the problem has been news organizations trying to report on technical documentation without understanding it. The WHO and the CDC have made press releases and public speeches/interviews that use such confusing jargon in a way that should be expected to be misunderstood and that is what bothers me the most.

The old studies of air droplets and aerosols that led to the 2 meter (6 foot) distancing advice were done in the lab, in still air. They were NOT done in rooms with multiple people in them and air currents being blown about by air conditioning, open windows, or people moving around. Or rooms where people were singing, chanting or speaking loudly to be heard over background noise.

It was a classic case of making false assumptions about real-world consequences based on research which was accurate for what it actually was, but which didn’t apply to the real world conditions. The same thing happened with mask-wearing - the studies on which they based the claim that cloth masks were ineffective did not look at the synergistic effects of everyone wearing masks, reducing the amount of infective material escaping infected people (rather than just looking at how well it prevents inhaling infected material), and reducing the distance the infected air travels from its source. When everyone wears masks and keeps their distance (more that 2 meters in lots of situations), masks and distance are very effective.

So yes, it became evident to anyone who has been paying attention to the new, pandemic-driven research into the effects of air movement that airborne transmission has been much more significant than expected, and that the 2 meter distancing advice, and a lot of the other advice, has been wrong and seriously inadequate.

This crisis has exposed some of the persistent flaws in the medical world, where studies may be perfectly good, but incorrect generalizations are made from them which are wrong. The medical establishment puts too much faith in their “educated guesses” - “if this is so, then that must be so”. Well, no. Assumptions like the ones that have been made are often very wrong. As we’re finding out once again.