https://judithcurry.com/2020/05/10/why-herd-immunity-to-covid-19-is-reached-much-earlier-than-thought/#more-26133
It's probably a LOT lower than the simplistic 1 - 1/R0 would suggest.
Why? Because social interactions are not homogeneous.
Simplistically, it's a graph theory problem. There are a small number of people with a large number of connections. These are potential super spreaders. But once these individuals are removed from the graph via immunity, R plummets.
One does not need, say, 60% of the population to be immune to have herd immunity. Rather, one needs 60% of the interactions in a population to involve someone with immunity, and given the social graph, that could be accomplished by a much smaller portion of the population, and will vary based on factors like density, mass transit, culture, etc.
The conclusions from the above paper:
It's probably a LOT lower than the simplistic 1 - 1/R0 would suggest.
Why? Because social interactions are not homogeneous.
Simplistically, it's a graph theory problem. There are a small number of people with a large number of connections. These are potential super spreaders. But once these individuals are removed from the graph via immunity, R plummets.
One does not need, say, 60% of the population to be immune to have herd immunity. Rather, one needs 60% of the interactions in a population to involve someone with immunity, and given the social graph, that could be accomplished by a much smaller portion of the population, and will vary based on factors like density, mass transit, culture, etc.
The conclusions from the above paper:
Quote:
Incorporating, in a reasonable manner, inhomogeneity in susceptibility and infectivity in a standard SEIR epidemiological model, rather than assuming a homogeneous population, causes a very major reduction in the herd immunity threshold, and also in the ultimate infection level if the epidemic thereafter follows an unconstrained path. Therefore, the number of fatalities involved in achieving herd immunity is much lower than it would otherwise be.
In my view, the true herd immunity threshold probably lies somewhere between the 7% and 24% implied by the cases illustrated in Figures 4 and 5. If it were around 17%, which evidence from Stockholm County suggests the resulting fatalities from infections prior to the HIT being reached should be a very low proportion of the population. The Stockholm infection fatality rate appears to be approximately 0.4%,[url=https://judithcurry.com/2020/05/10/why-herd-immunity-to-covid-19-is-reached-much-earlier-than-thought/#_edn20][20][/url] considerably lower than per the Verity et al.[url=https://judithcurry.com/2020/05/10/why-herd-immunity-to-covid-19-is-reached-much-earlier-than-thought/#_edn21][21][/url] estimates used in Ferguson20, with a fatality rate of under 0.1% from infections until the HIT was reached. The fatality rate to reach the HIT in less densely populated areas should be lower, because R0 is positively related to population density.[url=https://judithcurry.com/2020/05/10/why-herd-immunity-to-covid-19-is-reached-much-earlier-than-thought/#_edn22][22][/url] Accordingly, total fatalities should be well under 0.1% of the population by the time herd immunity is achieved. Although there would be subsequent further fatalities, as the epidemic shrinks it should be increasingly practicable to hasten its end by using testing and contact tracing to prevent infections spreading, and thus substantially reduce the number of further fatalities below those projected by the SEIR model in a totally unmitigated scenario.
