Caballus
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Fair enough; let's go with 60% for the sake of discussion. The question begged then is how long it would take to get to 60% and what the estimated loss would be en route to that number under various circumstances. To be useful for assessing risk in practical terms, such a calculation would have to assume that there is no vaccine, estimate when a reliable anti-body test will be available, and recognize that we do not yet know how long COVID immunity lasts or how robust any immunity really is...no?It is down to maths. (R0 - 1)/R0 is the calculation, so with an R0 of 2.5 it is 60% and R0 of 3 is 66%, R0 of 3.5 is 71% for herd immunity.
It is however a lot more complicated than that as firstly the R0 will be different in different subsets of populations nationally (such as in care homes) and also as more people become immune the available hosts fall so the R0 falls - which is why when 60% of the population have antibodies the R0 falls to 1. It isn't a switch at 60%, it is a gradual decrease from 2.5 to 1 between 0% and 60%
For diseases such as measles with an R0 of as much as 18 you need 94% of the population immune to stop logarithmic increases.
The R0 for COVID 19 is probably around 2.5 but maybe a little more
https://www.cebm.net/covid-19/when-...tion-to-viral-reproduction-numbers-r0-and-re/
So for example in NY, even left uncontrolled now the R0 will be lower than in a state with a naive population, it will still be more than 1 though.
Another important factor is that in most virus outbreaks the pathogenicity tends to fall with time. This is partly mutations, but primarily that the more severe strains get transmitted less easily as the victims are often in hospital and die meaning they can't pass it on. Milder asymptomatic strains are easily spread as people don't know they have it and spread it around. It also explains why a lot of health workers have the more serious strains as they are working in the places the more severe cases are present. They also get a higher viral load which is known to increase the severity of disease.
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