Why do some COVID-19 patients infect many others, whereas most don’t spread the virus at all?

https://www.sciencemag.org/news/2020/05/why-do-some-covid-19-patients-infect-many-others-whereas-most-don-t-spread-virus-all?fbclid=IwAR3KUGIRXl_Fh_pNWnp8gkr7NOwTj5II8Atja40_pj5PKNTQN9_kLrpAgrE

By Kai KupferschmidtMay. 19, 2020 , 5:25 PM

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Other infectious diseases also spread in clusters, and with close to 5 million reported COVID-19 cases worldwide, some big outbreaks were to be expected. But SARS-CoV-2, like two of its cousins, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), seems especially prone to attacking groups of tightly connected people while sparing others. It’s an encouraging finding, scientists say, because it suggests that restricting gatherings where superspreading is likely to occur will have a major impact on transmission, and that other restrictions—on outdoor activity, for example—might be eased.

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Why coronaviruses cluster so much more than other pathogens is “a really interesting open scientific question,” says Christophe Fraser of the University of Oxford, who has studied superspreading in Ebola and HIV. Their mode of transmission may be one factor. SARS-CoV-2 appears to transmit mostly through droplets, but it does occasionally spread through finer aerosols that can stay suspended in the air, enabling one person to infect many. Most published large transmission clusters “seem to implicate aerosol transmission,” Fraser 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.

The factor scientists are closest to understanding is where COVID-19 clusters are likely to occur. “Clearly there is a much higher risk in enclosed spaces than outside,” Althaus says.

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Some situations may be particularly risky. Meatpacking plants are likely vulnerable because many people work closely together in spaces where low temperature helps the virus survive. But it may also be relevant that they tend to be loud places, Knight says. The report about the choir in Washington made her realize that one thing links numerous clusters: They happened in places where people shout or sing. And although Zumba classes have been connected to outbreaks, Pilates classes, which are not as intense, have not, Knight notes. “Maybe slow, gentle breathing is not a risk factor, but heavy, deep, or rapid breathing and shouting is.”

Timing also plays a role. Emerging evidence suggests COVID-19 patients are most infectious for a short period of time. Entering a high-risk setting in that period may touch off a superspreading event, Kucharski says; “Two days later, that person could behave in the same way and you wouldn’t see the same outcome.”

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