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Ten percent of Covid-19 cases spread 80 percent of the infection

Researchers are puzzled by the fact that most people infected with SARS-CoV-2 do not seem to spread the infection, while a few are so-called "super-spreaders" that infect dozens.

Published: June 4, 2020, 8:46 am

    “Probably around 10 percent of cases account for 80 percent of the spread,” says mathematician and epidemiologist Adam Kucharski of the London School of Hygiene & Tropical Medicine.

    The researchers are still trying to understand what sets super-dispersers apart from others. It has been found that some environments are more dangerous than others. Many people have been infected in places where many gather indoors and expel large volumes of breath, such as when singing or screaming.

    “It is possible that slow, gentle breathing is not a risk factor, while deep, vigorous or rapid breathing and shouting are,” says Gwenan Knight, also at London University.

    Also, what stage of the disease you are in is of great importance for the spread. It seems that those infected by the Coronavirus are only really infectious for a short time.

    “Two days later the same person could behave in the same way and it would not be the same result,” says Kucharski.

    Because of this peculiarity in the spread of Covid-19, it has been observed that clusters of infected are formed. This makes it easier to contact and find a common source.

    A Japanese study has found that the risk of infection indoors are 19 times higher than outdoors since aerosols are droplets smaller than 5 micrometers which float in the airstream. The viral load is thus diluted in open air.

    Other infectious diseases also spread in clusters. But Covid-19, like SARS and MERS, seems especially prone to attacking clusters. Therefore, restricting gatherings where superspreading is likely to occur will have a positive impact on transmission.

    Most of the discussion around the spread of SARS-CoV-2 has concentrated on the average number of new infections caused by each case. This reproduction number (R) is about three without social distancing. But in real life, some people infect many others and others don’t spread the disease at all.

    One researcher told Science: “The consistent pattern is that the most common number is zero. Most people do not transmit.”

    In addition to R, scientists therefore also calculate 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, the 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 while current estimates of the dispersion factor k for SARS-CoV-2 vary between 0,1 and 0,5.

    That means that cluster infections from relatively few superspreading events drive the epidemic more than single transmissions from one person to another person.

    In Japan, people have been told to avoid environments with what it calls the “Three Cs”, meaning close contact in closed-off, crowded spaces, instead of a total lockdown.

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