Citat:
Intressant ändå att de anser att det dog ut. Man brukar ju annars anse att det räcker med en person, som i sin tur smittar två, som smittar 4, som smittar 8, osv för att en epidemi ska utvecklas. Det verkar som om R0 inte alls var 2 som man trott, i alla fall inte i ett mindre samhälle som Svärdsjö (1250 inv.)
Jo, R0 ligger runt 2. I artikeln i Science, så uppskattas R0 till 3. Men R0 är ett genomsnitt. Det är inte så att varje smittbärare infekterar ytterligare 2 personer. Med viruset bakom Covid-19, så finns väldigt stora skillnader mellan individer. Dom flesta smittbärare smittar inte någon, medan vissa smittbärare smittar desto fler. Dom smittbärare som smittar många, drar upp det genomsnittliga reproduktionstalet.
Jag borde kanske ha citerat en större del av artikeln i Science.
Från Science:
Why do some COVID-19 patients infect many others, whereas most don’t spread the virus at all?Ifall ett land får in viruset, så finns en stor sannolikhet att man har tur, och ingen superspridning sker. I så fall håller sig det genomsnittliga reproduktionstalet lågt, och smittspridningen dör ut. Men ifall superspridning sker, så tar epidemin fart.
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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.
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They concluded that k for COVID-19 is somewhat higher than for SARS and MERS. That seems about right, says Gabriel Leung, a modeler at the University of Hong Kong. “I don’t think this is quite like SARS or MERS, where we observed very large superspreading clusters,” Leung says. “But we are certainly seeing a lot of concentrated clusters where a small proportion of people are responsible for a large proportion of infections.”
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That could explain some puzzling aspects of this pandemic, including why the virus did not take off around the world sooner after it emerged in China, and why some very early cases elsewhere—such as one in France in late December 2019, reported on 3 May—apparently failed to ignite a wider outbreak. 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. If the Chinese epidemic was a big fire that sent sparks flying around the world, most of the sparks simply fizzled out.
Från Science:
Countries that have beaten back the virus to low levels need to be especially vigilant for superspreading events, because they can easily undo hard-won gains. After South Korea relaxed social distancing rules in early May, a man who later tested positive for COVID-19 visited several clubs in Seoul; public health officials scrambled to identify thousands of potential contacts and have already found 170 new cases.Du får läsa hela artikeln i Science. Jag vågar inte citera en större del än jag redan har gjort, med tanke på reglerna kring upphovsrätt.
If public health workers knew where clusters are likely to happen, they could try to prevent them and avoid shutting down broad swaths of society, Kucharski says. “Shutdowns are an incredibly blunt tool,” he says. “You’re basically saying: We don’t know enough about where transmission is happening to be able to target it, so we’re just going to target all of it.”
