Thread by @yaneerbaryam

Imagine a country that has reduced their number of cases to only a few, say 5 infectious individuals, in a population of 5 million. A person might say: “The likelihood of getting infected today is only about 1 in a million. That is so unlikely that I won’t take precautions.” 1/

And no precautions would be taken by that individual on that day. If everyone follows that rule, then someone will be infected. The number of infected individuals will go up. Each infectious person infects 3-5 others, they infect a few others, and there are more and more cases.

By the end of the first week there are about 50 cases, 500 after two weeks, 5,000 after three weeks. By then the odds have changed from 1 in 1 million, to 1 in 100,000, to 1 in 10,000, to 1 in 1,000. When do people decide to take precautions?

At some point drastic action is necessary. Then the cases go down. At some point the country gets to the situation where people say, "OK now there is only one in a million chance of being infected." Again the infections will grow. How do we stop this cyclic process?

1 Patience—People in general have to recognize that the only way to stop is to get to zero so that everyone takes some precautions until there are zero cases. Getting to zero takes another week or two, depending on the strength of actions taken.

2 Early case detection with contact tracing—Everyone has to participate by quickly identifying early symptoms. Contact tracing teams identify likely contacts, they quarantine and test both symptomatic and asymptomatic contacts (of order 100 of them).

3 Green Zone strategy—take precautions where cases are, and not in areas where they aren’t. Relaxation of restrictions by geographical region, not business type or profession. This requires restrictions on nonessential travel between zones, so infections don't go between zones

Areas with the disease have a higher chance of infection so people naturally take strong action. In areas without cases people don’t need to follow restrictions. Because of zoning, the number of cases is taken relative to the local population instead of the national population.

For example, 5 cases in a county of 50,000 in a nation of 5M population. Instead of 5M people being concerned only a little (5/15M), we have 50K alert people with (5/50K = 1 in 10,000) and 4.950M people living an almost ordinary life (including normal economic activity),…

…but following zone travel restrictions until the last zone is cleared.

endcoronavirus.org/papers/psychol…

Fix:

For example, 5 cases in a county of 50,000 in a nation of 5M population. Instead of 5M people being concerned only a little (5/5M), we have 50K alert people with (5/50K = 1 in 10,000) and 4.950M people living an almost ordinary life (including normal economic activity),…