It’s not R.
www.theatlantic.com
Because of overdispersion, most people will have been infected by someone who also infected other people, because only a small percentage of people infect many at a time, whereas most infect zero or maybe one person. As Adam Kucharski, an epidemiologist and the author of the book
The Rules of Contagion, explained to me, if we can use retrospective contact tracing to find the person who infected our patient, and
then trace the forward contacts of the infecting person, we are generally going to find
a lot more cases compared with forward-tracing contacts of the infected patient, which will merely identify
potentialexposures, many of which will not happen anyway, because most transmission chains die out on their own.
The reason for backward tracing’s importance is similar to what the sociologist Scott L. Feld called
the friendship paradox: Your friends are, on average, going to have more friends than you. (Sorry!) It’s straightforward once you take the network-level view. Friendships are not distributed equally; some people have a lot of friends, and your friend circle is more likely to include those social butterflies, because how could it not? They friended you and others. And those social butterflies will drive up the average number of friends that your friends have compared with you, a regular person. (Of course, this will not hold for the social butterflies themselves, but overdispersion means that there are much fewer of them.) Similarly, the infectious person who is transmitting the disease is like the pandemic social butterfly: The average number of people they infect will be much higher than most of the population, who will transmit the disease much less frequently. Indeed, as
Kucharski and his co-authors show mathematically, overdispersion means that “forward tracing alone can, on average, identify at most the mean number of secondary infections (i.e. R)”; in contrast, “backward tracing increases this maximum number of traceable individuals by a factor of 2-3, as index cases are more likely to come from clusters than a case is to generate a cluster.”
denne logikken forteller oss vel at det er henimot meningsløst å sette en drøss av folk automatisk i karantene fordi det var en sjuk en på en plass blant dem?