From Scientific American (June 23, 2020):

In fact, research on actual cases, as well as models of the pandemic, indicate that between 10 and 20 percent of infected people are responsible for 80 percent of the coronavirus’s spread.

Researchers have identified several factors that make it easier for superspreading to happen. Some of them are environmental.

• Poorly ventilated indoor areas seem especially conducive to the virus’s spread – A preliminary analysis of 110 COVID-19 cases in Japan found that the odds of transmitting the pathogen in a closed environment was more than 18 times greater than in an open-air space.
• Places where large numbers of people congregate – As a group’s size increases, so does the risk of transmitting the virus to a wider cluster. A large group size also increases the chance that someone present will be infectious.
• The longer a group stays in contact, the greater the likelihood that the virus will spread among them – The benchmark used for risk assessment in her contact-tracing work is 10 minutes of contact with an infectious person, though the CDC uses 15 minutes as a guideline.
• Some activities seem to make it easier to spread respiratory gunk – Speech emits more particles than normal breathing. And emissions also increase as people speak louder. Singing emits even more particles, which may partially explain the superspreader event at the Washington State choir practice. Breathing hard during exercise might also help the spread of COVID-19.

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