In a previous post I talked about R, the reproduction number for an infectious agent such as SARS-COV2. R is the average number of other people a person with infection infects. The basic idea is that if R is less than one, the infection dies out. This works for infections like influenza which spreads pretty much evenly in susceptible populations.
There is another epidemiologic number called k, which is the dispersion factor. The math is somewhat technical, but if k is substantially less than one that means that the infection is spread primarily by a small number of superspreader events. It is estimated that k for SARS-COV2 is about 0.1
As we learn more about how this virus spreads, we have discovered that about 80% of people who are infected are unlikely to infect anyone else. The virus is spread primarily by “superspreader events” where one infected person is responsible for as many to as 50 or 100 infections. In other words, about 20% of infected people are responsible for the majority of the spread of the pandemic.
Superspreader events are relatively rare, so if we could focus on preventing the conditions that lead to super spreader events, then we could get the pandemic under much better control without having to shut down the whole economy.
There are some well known superspreader events. The Biogen leadership conference in Boston led to infection of 99 people in Massachusetts alone. A funeral in Albany Georgia led to infection of more than 100 people. Infection in a single person in a choir practice in Washington state led to 52 infected people, two of whom died. In Arkansas an infected pastor and his wife infected 30 church member at services over several days. Three of them died. An additional 26 people were infected from church members, one of whom died.
There are common factors in each of these events. They all happened in large groups of people who were close together, mostly in poorly ventilated indoor spaces. They involved people who were talking, often loudly, or singing or eating (you can’t wear a mask while you are eating or drinking). Talking or singing produces lots of particles and are likely to aerosolize (tiny droplets hanging in the air) the virus and make it much more likely to spread. It also turns out that exercising vigorously in a closed space, such as a gym also can cause superspreader events. For example fitness dance classes in South Korea with up to 22 people in a room led to 65 new cases. On the other hand, yoga classes were not associated with new cases.
So how do we prevent superspreader events? Obviously by avoiding situations where large groups are close together in enclosed spaces eating talking or singing. That means no indoor dining in most restaurants, no exercising in gyms in groups larger than 5-10 people, and no church services indoors. It also means avoiding indoor bars. Alcohol disinhibits people and makes them less careful. It means prohibiting crowds of spectators at sporting events or concerts.
Large indoor retail spaces where it is possible to keep social distancing and wear masks are unlikely sources of super spreading events. Time of contact is important too. If you are in contact with an infected person for less than 5 minutes, you are unlikely to become infected. That means passing people briefly indoors or out is unlikely to create a superspreader event.
People have also worried about using public bathrooms. Although it is true that public bathrooms are for the most part poorly ventilated enclosed spaces, no one stays there very long. Even though flushing a toilet creates a plume of particles, they don’t hang in the air very long. As far as we know, there has been no documented case of infection due to using a public bathroom.
An excellent article that goes into more detail about super spreading events and their importance can be found in an article in Scientific American. It is written in non technical language and is easy to understand. Here is a link to that article.
Bottom Line
COVID-19 infections are primarily transmitted by superspreader events. Eighty percent of infected people do not infect others. The vast majority of infections that maintain the spread of infection are produced by twenty percent of infected people. Superspreader events are associated with large groups of people in closed indoor spaces where talking, singing, eating and drinking, and vigorous exercise are occurring.