| Breaking down the New York City report, Cuomo noted that the Bronx had the highest infection rate of the five boroughs, with a staggering 27.1% of respondents having COVID-19 antibodies. ... “That number remains obnoxiously and terrifyingly high,” Cuomo said. “It’s not where we want it to be.” Source |
Epidemics don't end with everyone in quarantine, because life cannot be sustained by a population in perpetual quarantine. Eventually, a quarantined population must get back to work. then people will have to come face to face once again, and the disease will resume it's spread.
So how do epidemics end? They end when the number of people with immunity to the disease rises to the point that the number of people each infected person infects falls below one. At that point, disease spread slows and dies.*
And what determines who has immunity? There are two key factors. One is the acquisition of immunity among those who have been infected and survived the disease. The other is the process of natural selection. Those who lack the necessary immune system genes to fend off the disease die, so that a higher proportion of the residual population has the genes that provide either immunity to infection, or the capacity for recovery from infection.
The seasonal flu is an example of a non-lethal disease that induces immunity in those infected. The plague, or black death, is a an example of a more or less lethal disease that kills many but not all that it infects, leaving a more resistant population more or less immune to the disease.
And, in the worst case, are diseases against which an entire population is vulnerable, and in that case the entire population dies, as was the fate of many Amerindian communities afflicted by diseases brought by explorers and colonists from Europe to the New World.
Covid-19, fortunately, is a relatively mild disease for most who are infected by it, leaving them with protective antibodies that reduce susceptibility to future infection. The Covid epidemics working theirs way through the nations of the world seem to be killing no more than 30 to 40 people per thousand of those infected (at least in the developed world), these being mainly the elderly with existing heart or respiratory diseases**.
The end to the Covid-19 pandemic will thus occur when the virus has gone through every population causing an increased level of immunity that halts the disease's spread.
What proportion of the population must have immunity before the rate of recovery from the disease exceeds the rate of infection is not something that can be exactly predicted, since it depends on the way people in each community interact. In a nation of hermits, the disease would never spread and there could be no epidemic. Otherwise, the particular features of specific communities, in particular the way in which people interact with one another, but also many other factors such as age distribution, air quality, nutrition, etc., determine the infection rate that must be attained before an epidemic dies.
What that means is that although lock-downs, quarantines, social distancing and face masks can slow the spread of disease, they have no effect on the incidence of the disease at which disease spread slows and ultimately stops.
So well done The Bronx. With 27% of the population reported to have antibodies for Covid19, they may not be there quite yet, but they are well on the way to the condition of so-called herd immunity, when immunity to the pathogen is sufficiently widespread to cause its spread to slow and die even after a return to normal life.
Despite what crackpots such as Ron Unz of the Unz Review, New York State Governor Cuomo and the political class generally, plus most public health officials tell you, the spread of Covid19 and hence of immunity to Covid19 is good news that heralds the return to normality. But don't expect to hear that from the gerontological medical experts, the politicians and the vendors of patent medicines and vaccines now in the limelight and positioned to profit by the present panic.
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* In New York City, the hardest hit place in North America if not the world, the number of new infections reported daily is down to about one third of the peak rate, indicating that, under present conditions of restricted social mobility, herd immunity has been achieved and the epidemic is dying. An end to the lock-downs in NY City may increase the infection rate if the percentage of the population with immunity is insufficiently high for herd immunity under the changed social dynamics. Certainly, an end to the lock-downs will cause a temporary increase in the infection rate, but a new downward trend leading to extinction of the virus will resume within a reasonably short time.
What percentage of the population must have immunity to achieve herd immunity is impossible to model accurately since people do not interact randomly, but in many distinctive ways according to factors such as age, profession, the need to commute, etc. Certainly, the percentage of infected people needed for herd immunity will be less than the theoretical 67% (assuming an R nought of 3.0), and quite possibly less than half that number.
** In the Bronx, with 27% having Covid-19 antibodies, the number infected is around 382,000 people, of whom 1,700 have died for an infection-specific mortality rate of 0.44%. Of those who have died, 545 were resident of nursing homes or adult care facilities, and most had other serious medical conditions. For the remainder of the population the infection-specific mortality rate has been 0.3%. (Source: Riverdale Press).
Related:
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