Tag Archives: SARS-CoV-2

Understating Airborne Covid-19

My title is slightly misleading but meant to tell you what this essay is about. I want to talk about recent reports that SARS-CoV-2, the virus that causes Covid-19, is airborne.

The concept of airborne in thinking about pathogens is probably the single most misunderstood thing in epidemiology, not by epidemiologists but by regular people. It is also probably the most evocative, and stupifying. Recently, the word “airborne” has been used in discussions of Covid-19, and this led to many extreme reactions. Like this:

Input: Bla bla bla Covid-19 bla bla bla airborne.

Output: All the credible experts have agreed that Covid is airbone! it is no longer spread by contact, but now it is airborne! The Fauci mutation probably made it airborne! And so on!

SARS-CoV-2 is spread by shedding from an infected person’s respiratory system and getting into a new host’s respiratory system via droplets of mucus that go from hand to hand, hand to mouth, mouth to hand, mouth or nose to surface, surface to hand then hand to mouth, etc. Human upper respiratory bodily fluids (snot, etc.) get on stuff and then people touch stuff and then it get into their respiratory system. This is how most cold and flu infections are passed on, generally. That is not airborne spread.

Among all the many viruses that give us colds or the flu — the many strains of influenza, rhinovirus, coronaviruses other than SARS-CoV-2, etc. — this is how infection happens.

Again, this is not airborne spread. It might be airborne in your head, because you imagine someone sneezing, into the air, droplets of virus-containing spittle and snot flying around in the air, and since that stuff flies through the air it must therefore airborne. But that is not what airborne means, and the distinction is important.

There is probably a certain amount of true airborne transmission in any of the above mentioned categories of virus, including the flu and more common colds. But it is rare enough that these diseases are not said to be airborne.

So what is airborne then, if it is not simply flying snot particles?

Airborne spread requires several things to be true often enough that an observable number of cases were spread in this way. First, the virus must be aerosolized. This means that the virus is embedded in a very small gobs of snot, perhaps near 5 microns, droplets that are small enough to be suspended in the air. Larger drops will fall out of the air, these smaller drops will float in the air like they were part of the air. They act like a gas in the air.

The droplets also have to be small enough to get into the parts of the respiratory system that the virus targets, which for SARS-CoV-2 is not too much of a limiting factor since it likes to inhabit the upper respiratory tract. But, since it also can invade the lungs, there would be the possibility that airborne transmission would be more associated with a more serious infection, if and when it happens.

Airborne spread also requires that the virus can live in the air long enough to get to its target. The longer the virus can live in the air, the worse of a problem it is because it can travel farther, through ventilation systems, down hallways, etc. There is no evidence that SARS-CoV-2 does that like, say measles, the king of airborne infection does it. Whatever environmental conditions are experienced by this suspended droplet have to NOT kill the virus. IV light kills SARS-CoV-2, so it is not going to get far during the day, outside, when the sun is out. SARS-CoV-2 might like certain humidity levels. None of this is really known for SARS-CoV-2, but it is a virus of type that we generally know about. Other forms of coronavirus are known to survive a while in the air, so that may pertain here. Be cautious in reacting to what you hear though. Detecting a virus some distance away from a sneeze does not mean that the virus is viable or capable of infection

Also, the virus has to be out there in the air in sufficient numbers to actually cause an infection. One would think that it only takes one single virus to infect someone, but generally it takes a much larger number. There seems to be a threshold for most viruses. The body dispenses of the first N viruses, then after that it gets harder, and eventually the system is overwhelmed. Maybe. The point is, virus experts will tell you that is has to be a large number for most viruses, and this is certainly true for SARS-CoV-2.

Truly aerosolized, viable, in sufficient numbers.

Finally, note that if SARS-CoV-2 was mainly airborne, we would know it by now. You can look at it, epidemiologically, and say, no, the main form of transmission is not airborne. That does not mean that there is not an airborne component, but it means that airborne is not the major way of spread. That has not changed.

What does the new research tell us?

Well, by the standards of peer reviewed scientific research, pretty much nothing, because that research is still in its infancy. But here is what happened. Several cases of infection have been reported that can be best explained by airborne infection. How many? So few that some would interpret that as potentially useless data. These may be cases that are simply misreported. Somebody licked someone else’s tongue and refuses to admit it. Does that really happen? Well, ask any expert on the epidemiology of sexually transmitted diseases about it. Of course it can. Most rare cases can be explained away or ignored.

But in this case, a large number of experts have settled on a provisional consensus: They see enough cases of possible airborne transmission of SARS-CoV-2 to ask the overarching institutional authorities like the CDC to seriously consider it and look into it. Yeah, that is it. Important, concerning, should shape policy modestly for now, requires more consideration. The smart money is on SARS-CoV-2 being transmittable via aerosol, though that will probably not be the main modality of transmission in most settings. That is my bet. Airborne transmission can happen, and will happen in some cases. More on that below. But, this is provisional.

What this does not mean.

This does not mean that there is a new mutation. Repeat: this is not a new mutation. This has been there all along, and the fact that it has not been obvious since the beginning means, as stated, this is not the new mode of transmission. This does not mean that the virus has changed. Probably.

This also has no impact on mask wearing. Airborne transmission will go right around the masks most people wear, but we already know that if airborne transmission is happening, it is not the main way the virus is spread. This is NOT AN ARGUMENT TO NOT WEAR MASKS so don’t go making that argument or you are a full-on jerk. Ignorant jerk. I know you won’t, but if you see that argument being made by others, that is what you are seeing. That argument is so stupid, you can expect Trump to make it soon.

What this might mean.

This is the important part of all this, worthy of careful consideration. Assume that normal near-distance non airborne transmission is the normal and most common form of transmission by a large margin. We assume that if people are kept a minimum of 6 feet away from each other (or 10 if you like) and do not share objects with their hands and faces, i.e, social distancing, that transmission will be minimized. This works for social gatherings, according to some, especially if masks are worn.

However, over longer term, while people are avoiding infecting each other by keeping their mucus to themselves, a low level background transmission via the air could be happening at a small level.

It would be rare. Say one hour of exposure within a single medium size room with modest air circulation has a one in a thousand chance of one infected person giving the disease to one other person in the room. (I am totally making up all these numbers, but just bear with me.)

But now, we take that room and put between zero and three infected people in it, and 30 target non infected people. But we put then in that room for 8 hours, and do that for 185 days. This configuration of people might sound familiar to you.

This is a classroom full of students social distancing. But wait, you say, if they are social distancing, they can’t fit 30 people in the room. But you would be wrong in some cases. Elementary schools with the pod system have four classes of 30 (including teachers) in the room. They will get their social distancing by spreading out into larger rooms in closed high schools or other places (gyms, etc.), so the main class of about 30 is still in one room. Maybe not. The point is, in the worst case scenario, we divide 1,000 by 8 (hours) then again by 185 (days) to get a baseline on transmission probability (though the math is slightly more complex than that) to arrive at this conclusion: Transmission within the classroom where there are one or two virus shedding individuals on any given day is nearly inevitable if there is a low probability of airborne transmission. Most classrooms may have zero infected people most of the time, but in a given school there would be several classes. In a given school system, maybe dozens and dozens.

If the air circulation does not remove the viruses, maybe they are being spread across the school. Students passing in halls, or any classes where the kids are reshuffled add to the dynamic, families with multiple kids (or both kids and staff) in the same school, etc. add to the dynamic.

You can do a similar calculation for restaurants and bars. Regular inside dining and bar hopping even with social distancing and mask wearing is probably not recommended if there is a low level of airborne transmission. More limitations on how retail shopping happens may be recommended. Certainly, unnecessary retail shopping maybe an unnecessary danger.

The final meaning of it all: When it comes to basic day to day life, under the current conditions of caution and distancing, this airborne problem would not have that much of an effect because it has to be rare. We know it is rare (if it is real) because if it was common we would see it. But, under school or large workplace reopening conditions, or reopening of indoor dining and shopping, etc., it may be a factor that causes two really bad problems.

1) More outbreaks, and some insidious ones. The school children, some getting very sick and maybe dying, others never becoming ill, passing the disease on to their families. Ignoring the airborne problem may involve asking our children to kill their grandparents, then live with that for the rest of their lives. You might get sick because you needed to shop for a new comic book or try out the headphones at the electronic store instead of ordering on line.

2) Not discussed anywhere else as far as I know, but I would think obvious: if we set up a situation where the rare airborne transmission has a better chance of actually transmitting the disease, we may also be setting up a positive selective environment for that. In other words, we may help make SARS-CoV-2 more airborne by giving it this chance. That is pure speculation on my part, but speculation based on some damn powerful theory (Darwinian evolution). It is not a chance I’d like to take.