COVID-19: Airborne transmission?
Some of you may have been as confused as I was by the recent reports suggesting that the possibility of airborne transmission of SARS-CoV2 has been neglected. You would be forgiven for thinking that the airborne route is what we’ve been talking about all along, and that it is the main route of transmission. That’s what masks and distancing are all about. This is an attempt to straighten out the issues.
When we cough or sneeze, or, to a lesser extent, talk or even just breathe, we eject small droplets of saliva and mucus. A sneeze may produce thousands of these, while a cough may yield only a few hundred. For talking, or singing, it will depend on the consonants – f, b, p, t and s are especially good at producing droplets (I am indebted to Cedric Mims in The Pathogenesis of Infectious Disease for the observation that most abusive words in English start with one of these consonants!). If we are infected with a respiratory tract pathogen, such as SARS-CoV2, then some of these droplets may contain virus particles. I’ll come back to that later.
What happens to those droplets? That depends on their size. The largest ones (1mm or more) will fall to the ground (or some other surface) within a few metres or less. With the smaller ones, because they have a large surface area relative to their volume, the water content will evaporate rapidly, resulting in tiny particles (‘droplet nuclei’) less than 5 micrometres in size (a micrometre is a millionth of a metre, or a thousandth of a millimetre). These can remain airborne virtually indefinitely. It is the potential of these particles to transmit the disease that is the focus of the current discussion. This is sometimes referred to as ‘aerosol transmission’ to distinguish it from transmission by large droplets.
Some unanswered questions
In order to assess the significance of the potential risk posed by these tiny particles, we need to consider the following factors:
– the likelihood that they carry the virus
– how many virus particles they carry
– how long will the virus remain able to establish infection
– how are they distributed in the air
– How many virus particles are needed to cause an infection (the infective dose).
Most of these are unknown; we’re just guessing, partly based on experience with other viruses. Let’s fill in some details.
Will the particles contain virus particles, and how many?
This depends on another unknown, the viral load, or more specifically the number of virus particles shed by someone infected. One of the main factors limiting our knowledge here, and elsewhere, is that almost exclusively the assays are based on detection of viral RNA rather than viable virus particles (which are much more difficult to measure).
Leaving that question on one side, it is intuitively obvious (and probably true) that the larger droplets are more likely to contain virus particles than smaller ones, and similarly are likely to have more virus in them. That’s just on a statistical basis. You should also consider the size of the virus. A single coronavirus, including the spikes, is about 130 nanometres (nm) in diameter (one nanometre being a thousandth of a micrometre). So if you put 7 or so viruses side by side, that would span the diameter of a one micrometre droplet. Without doing the detailed calculation, it can be estimated that the maximum capacity of the largest of these persistent airborne droplets (say 5 micrometres) might be of the order of 100 virus particles, if they can be tightly packed. And many of the droplets will be much smaller, containing no more than a few viruses. This will be relevant when we look at the infectious dose later.
How long does the virus remain infectious in small droplets?
Another unknown, and here the RNA assay is of no help at all. RNA will remain long after the virus itself is ‘dead’. But it is likely that it will be of limited duration due to drying – and, especially in the open air, the effect of UV light.
How are the droplets distributed?
This is a bit easier to answer. For the larger droplets, which are essentially transmitted directly from the source to the subject, the likelihood is dependent on the distance separating them, and more or less to the square of that distance (as it can go to either side of you). If the smaller droplets are distributed throughout the air-space, it will be related to the size of the room you are in. If you have a lot of people in a small, low ceilinged, room, then the risk may be considerable. If the room is larger it will be much less so, especially if the room is well ventilated. And of course if you are outside, the risk virtually disappears.
How likely are you to be infected?
This depends on the biggest unknown of them all, the infectious dose (i.e., how many virus particles do you need to inhale in order to catch the disease?). This varies a lot from one disease to another, from a few hundred up to millions. Because we have a variety of non-specific mechanisms protecting us against invading microbes, very few diseases have infectious doses less than a few hundred – it is often said that tuberculosis can arise from a single bacterium penetrating as far as the lungs, but that is exceptional (and a bit dubious!). For the original SARS virus, the infectious dose has been estimated as a few hundred (although this is not much more than a guess, and it may be higher), so it is often assumed that this will apply to SARS-CoV2 as well. If we combine that estimate with the previous discussion, recognising that each of these tiny droplets is likely to contain only a small number of virus particles, we would have to inhale dozens or hundreds of such particles. Potentially, this could happen in a small, ill-ventilated room if there was someone shedding large numbers of virus, but my conclusion is that this is likely to have a comparatively small effect on the overall transmission of the virus, compared to the risk of more direct transmission by larger droplets.
10 July 2020