When 61 people met for a choir practice in a church in Mount Vernon, Washington, on 10 March, everything seemed normal. For 2.5 hours the chorists sang, snacked on cookies and oranges, and sang some more. But one of them had been suffering for 3 days from what felt like a cold—and turned out to be COVID-19. In the following weeks, 53 choir members got sick, three were hospitalized, and two died, according to a 12 May report by the U.S. Centers for Disease Control and Prevention (CDC) that meticulously reconstructed the tragedy.
Many similar “superspreading events” have occurred in the COVID-19 pandemic.
Most of the discussion around the spread of SARS-CoV-2 has concentrated on the average number of new infections caused by each patient. Without social distancing, this reproduction number (R) is about three. But in real life, some people infect many others and others don’t spread the disease at all. In fact, the latter is the norm, Lloyd-Smith says: “The consistent pattern is that the most common number is zero. Most people do not transmit.”
That’s why in addition to R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k is, the more transmission comes from a small number of people.
A large K means it spreads easily between all people rather than via tight clusters of people. The 1918 pandemic was 1.0 which is a large K meaning everyone was a spreader. In contrast, the SARS-cov-1 and MERS viruses of the early 2000s had K of only 0.12 and 0.25 respectively, meaning they did NOT spread generally but rather in clusters.
Our beloved SARS-COV-2 (COVID-19) appears to be akin to its relatives SARS and MERS, with a K value possibly as low as 0.1
If k is really 0.1, then most chains of infection die out by themselves and SARS-CoV-2 needs to be introduced undetected into a new country at least four times to have an even chance of establishing itself, Kucharski says.
Individual patients’ characteristics play a role as well. Some people shed far more virus, and for a longer period of time, than others, perhaps because of differences in their immune system or the distribution of virus receptors in their body. A 2019 study of healthy people showed some breathe out many more particles than others when they talk. (The volume at which they spoke explained some of the variation.)
Singing may release more virus than speaking, which could help explain the choir outbreaks.
People’s behavior also plays a role. Having many social contacts or not washing your hands makes you more likely to pass on the virus.
Related story: UK Superspreader identified in early February 2020