What is sometimes called a 90/10 or 80/20 rule or the Pareto principle, crops up a lot in life. If you doing are research, 90% of the best results come from 10% of your effort (sadly it is typically not possible to work out in advance which 10%!). It also seems to apply to the COVID-19 pandemic. 80% of the infections come from from 20% of the infected carriers. As I talked about in the previous blog post, there are superspreading events, in which one infected person can infect tens of other people, in one day. At the other end of the spectrum, many infected people do not pass the virus on to even one other trticperson. With colleagues I am working on understanding how masks work, which leads to the question: Can wearing mask use reduce the number of these superspreading events?
This is hard question to answer. The superspreading events are associated with an infected person sharing space with a lot of other people, for example at a wedding. There are two obvious possibilities for the transmission to many people during the wedding. The first is that the infected person mingled during the wedding, so breathing directly on many other people. That’s possible, mingling and talking to many people often happens at these events.
The second is that the room was not well ventilated and that the infected person just breathed out a lot of virus, filling the room with small virus-containing droplets, that the wedding guests inhaled.
Now biology in general, and virus transmission in particular, is complex, so it may be a combination of both, but nonetheless it is useful, to think about these two mechanisms for transmission.
Now wearing masks should reduce but not eliminate the risk of transmission via both mechanisms*, although it should help more with the first than the second. Transmission between two people talking to each other within a metre or so seems likely to involve virus being transmitted in relatively large droplets which masks filter out efficiently, while transmission over several metres must be more via much smaller droplets that are filtered out less efficiently by masks.
But in either case, masks don’t eliminate the risk. Some infected people produce huge amounts of virus, and a simple cotton or surgical mask won’t filter it all out. Wearing masks is not about making you completely safe — the virus is tiny, 100 nanometres across, and so impossible to filter out with 100% efficiency — it is just one part of a set of measures to keep the number of people infected as low as possible.
* For other ways of reducing the risk of transmission will be much more or less effective, depending on how the virus is transmitted. If it is mostly at short-range then social distancing works well to reduce risk, whereas if it is more via the room’s air then turning over the air faster is the thing to do. Open windows, crank up any air conditioning, etc.
Chance & Necessity 
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