COVID-19 Transmission: Questionable Statements

How is COVID-19 transmitted from one person to another? If we know the answer to this question, we can make decisions on how to avoid getting infected.

A lot of people and organizations state that they know a lot about this - but if you actually bother to look at the science in detail, the picture is a lot less clear. Very early on in the COVID-19 epidemic, "experts" told us that the main way that COVID-19 spreads is by "droplets": an infected person coughs or sneezes, which spreads lots of droplets that contain the virus. You then get infected if you catch a droplet, or by touching a surface where a droplet landed.

A lot of the measures aimed at containing COVID-19 are based on this theory: wash your hands often, in case you touched a droplet; stay 6 feet away from others, since droplets don't travel that far; and more. Please don't get me wrong: these are definitely things you should do, because they definitely help to reduce COVID-19 transmissions. But are they sufficient? Considering that we still have almost as many daily new COVID-19 cases in the US as we had a month ago, that seem questionable. What is missing?

Let's look at some of the statements that are made to illustrate that COVID-19 "almost exclusively spread by droplets", as one recent article claimed. I will start with statements by made in a post that has been distributed widely, but I'll use it merely as an example of what is often stated in similar form.

The post makes several dramatic statements about coughs and sneezes:

"A single sneeze releases about 30,000 droplets, with droplets traveling at up to 200 miles per hour. "

"If a person is infected, the droplets in a single cough or sneeze may contain as many as 200,000,000 (two hundred million) virus particles which can all be dispersed into the environment around them."

But while the article is generally good about providing links to underlying science, it does not give a link to the claim that a single cough or sneeze might distribute two hundred million virus particles. I have tried to find any source for this information on Google Scholar and Pubmed without success.

The post also mentions that breath releases droplets:

"A single breath releases 50 - 5000 droplets. Most of these droplets are low velocity and fall to the ground quickly. There are even fewer droplets released through nose-breathing. Importantly, due to the lack of exhalation force with a breath, viral particles from the lower respiratory areas are not expelled."

Again, no source is given. Worse, this statement contains several false statements. The claim that droplets in exhaled breath "fall to the ground quickly" contradicts what is known about droplets in breath. The overwhelming majority of droplets in breaths is very small; one study described that 82% of all droplets in breath are less than 0.5 micrometers in diameter. Typically, droplets smaller than 5-10 micrometers are called "aerosols": due to their small size, they settle very slowly, and can remain airborne for long times. The second false statement is that "viral particles from the lower respiratory area are not expelled". This is contrary to the current understanding that breath aerosol droplets are formed "through fluid film rupture in the respiratory bronchioles in the early stages of inhalation", and the observation that deep exhalation can increase the number of breath droplets five fold.

Next, it gets interesting, when the author writes:

"We don't have a number for SARS-CoV2 yet, but we can use influenza as a guide. Studies have shown that a person infected with influenza can releases up to 33 infectious viral particles per minute. But I'm going to use 20 to keep the math simple."

That's pretty much it: 200 million  virus particles in a cough or sneeze, and just 20 in a breath. So every idiot can see that coughs and sneezes matter a million times more, right?

Not so fast! First of all, this does not really make sense in view of studies that show that many COVID-19 transmissions happen before the first symptoms show up. Especially in countries with strong contact tracing, pre-symptomatic or asymptomatic transmissions account for most of the new infections - and by definition, pre-symptomatic patients do not cough or sneeze.

So let is look at the actual scientific studies here. One study did indeed report up to 20 influenza virus particles in breath. But a second and larger study found much higher viral loads - up to 430 million virus particles when breath was collected for 30 minutes (which amounts to more than a million virus particles per breath). When looking only at patients who neither coughed nor sneezed during the 30 minutes their breath was collected, the maximum number of virus particles collected was still 370,000.  The detection was based on PCR, but the authors also showed that they could actually culture the virus they had collected, so the virus was still "alive".

But it gets better. There is also a study that specifically looked at influenza virus in coughs. This study found a average of only 16 viral copies per cough, and a maximum of about 140. These numbers are actually lower than the number of influenza particles found in breath!

The studies were done in different labs with different equipment, which we have to keep in mind when comparing numbers directly; but both studies found very large variations within the tested patients, with some patients having several orders of magnitude higher viral loads in coughs and breath than others.

Studies have shown that various activities can increase the number of droplets in exhaled breath substantially, often more than 10-fold. These include:

• Speaking (the louder, the more)
• Deep breathing (can produce more particles than coughing, the study found)
• Rapid exhalation

One activity that combines these is singing. Many "superspreader" events have been linked to churches and choirs; some even observed social distancing rules, but still resulted in multiple transmissions. Another activity that leads to deep breathing is heavy work, like in slaughterhouses - another common hotspot for COVID-19 superspreader events.

In view of the scientific knowledge, going to a gym where the goal is to breathe deeply sounds like a great way to get a COVID-19 infection.

There is substantial evidence that aerosol transmission can play a significant role in COVID-19 epidemics. Large, visible "droplets" from coughs and sneezes mostly fall to the ground within a short distance; but smaller droplets can remain in the air much longer. How much this increases the chances of getting infected depends on many factors: Inside or outside? How much air movement is there? Which direction is the air moving? How warm is it? How many people are there? How close are you to the infected person? How infectious is he or she?

Note that the (incomplete) list of factors includes proximity. Even with aerosol transmission, the change of getting infected is much higher if you get close to another person. If you are within a couple of feet from another person, you are "sharing breath" with this person: with every breath, you inhale some of the air he just exhaled. If he is infected, that creates the most direct and efficient way to transmit the virus: directly from his infected lung into your soon-to-be infected lung, with just a very short period in between. I think that this actually may be a, or perhaps the, major way that transmissions happen when people great each other with kisses on the cheek. Sure, a kiss on the cheek will probably transfer some virus particles onto the cheek - but that's not an infection yet. The virus still has to make it from the cheek into the mouth, nose, or eye before it can replicate, and in the mean time, it is subjected to temperatures where it does not survive very long. Even if you touch your check and then your mouth and thereby infect yourself, the virus still needs to get to a tissue where it can multiply, like the back of your throat, or your lungs.

So, what do we make of this all? First of all, you still need to be concerned about transmission by sneezing, coughing, or "fomites" (I like the German word "Schmierinfektion" better).  Just because aerosol transmission can play a role does not mean that the other transmission modes are not important (and perhaps even more important).

But if we know that breathing can create infectious droplets; that speaking, singing, and deep breathing creates more droplets; and small droplets can travel further than 6 feet, we can take that into account when around others. If I had to work in a big office or close to others, I would wear a well-fitting face mask the entire times, and probably have a couple of extras around to replace it from time to time. Fortunately, I can work from home, so the next example may be more relevant: when talking to friends on the beach on a windy day, I'll try to have both of us stand sideways to the wind (and more than 6 feet apart). I certainly don't want you to talk to me when I am downwind of you! I'll also keep my singing down, but that's no big change :-)