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Wed May 20, 2020, 07:38 AM

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

"Probably about 10% of cases lead to 80% of the spread."

Adam Kucharski, London School of Hygiene & Tropical Medicine

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. In a seminal 2005 Nature paper, Lloyd-Smith and co-authors estimated that SARS—in which superspreading played a major role—had a k of 0.16. The estimated k for MERS, which emerged in 2012, is about 0.25. In the flu pandemic of 1918, in contrast, the value was about one, indicating that clusters played less of a role.

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. Researchers in China studying the spread of the coronavirus outside Hubei province—ground zero for the pandemic—identified 318 clusters of three or more cases between 4 January and 11 February, only one of which originated outdoors. A study in Japan found that the risk of infection indoors is almost 19 times higher than outdoors. (Japan, which was hit early but has kept the epidemic under control, has built its COVID-19 strategy explicitly around avoiding clusters, advising citizens to avoid closed spaces and crowded conditions.)

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.”


A lot of discussion recently about transmission dynamics, most of which are extrapolated from viral loads & estimates. What does contact tracing/community testing data tell us about actual probability of #COVID19 transmission(infection rate), high risk environments/age?

15/ In summary:
While the infectious inoculum required for infection is unknown, these studies indicate that close & prolonged contact is required for #COVID19 transmission. The risk is highest in enclosed environments; household, long-term care facilities and public transport.

16/ High infection rates seen in household, friend & family gatherings, transport suggest that closed contacts in congregation is likely the key driver of productive transmission. Casual, short interactions are not the main driver of the epidemic though keep social distancing!

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Reply Why do some COVID-19 patients infect many others, whereas most don't spread the virus at all? (Original post)
octoberlib May 2020 OP
samnsara May 2020 #1
octoberlib May 2020 #2

Response to octoberlib (Original post)

Wed May 20, 2020, 07:51 AM

1. as a child i was a carrier for measles.. i just didnt ever get it yet i must have infected my

...siblings. Also...and im 69..i dont have smallpox scar because it wouldnt take. Twice the dr tried. They said I had my mothers antibodies for it. Thank goodness smallpox was contained shortly after. So maybe some other antibodies are causing some ppl not to get sick with it? Im taking NO chances!

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Response to samnsara (Reply #1)

Wed May 20, 2020, 08:06 AM

2. Yes, they're finding this with COVID , too. I read somewhere last week, maybe in WaPo

about an ER doctor who was amazed at the people who weren't getting infected at all and they lived in same household as the infected person and had prolonged, close contact with them.

Also they're only infectious or at peak viral load for a short period of time.
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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