Dear Reader:

About herd immunity, the concept is this: in a herd of elk, for example, a vulnerable, weaker elk in the middle of the herd is protected from an approaching predator by the stronger elk around him. Similarly, a person who is vulnerable to being infected by a  virus (a person who is not naturally immune to the virus by having developed antibodies for it following a previous infection, or a person who is not artificially immune to the virus by having been vaccinated), is protected from the virus if enough immune people (naturally or artificially) are surrounding the vulnerable individual.

The herd protection of a weak elk can be reached if enough stronger elk surround the weak elk, preventing an approaching mountain lion from accessing the weak elk. The herd protection of a person who is not immune to a virus can be reached if enough immune people surround the un-immune individual, preventing an approaching virus from accessing the individual that is not immune to  it.

The more contagious the virus, the more people are needed to be immune to the virus, for the herd protection to apply. The measles are very contagious and therefore, 90-95% of people within the herd aka population, need to be immune to it so that the  herd protection applies. Polio, almost as contagious, requires 80-85% of people within a population to be immune, so that the herd protection applies.

Based on early estimates of Cov-2, the virus that causes Covid-19, it may be that at least 70% of the population (according to John Hopkins School of Public Health epidemiologists) needs to be immune to it, so that the herd protection applies.

Another problem with herd immunity is that some viruses mutate so quickly that a person’s antibodies from a year before don’t work for the year after, or a vaccine from the year before doesn’t work for the year after, as is often the case with the flu.

The plan regarding Covid-19 is to develop a vaccine and make it available to the masses. When enough people within a population get vaccinated (70% perhaps), then herd protection will be reached. Mass vaccination may take place in 2022. Before that time, some people and countries are thinking about achieving the herd protection via natural immunity, meaning that enough people recover from the infection and develop antibodies to prevent a second infection.

The hope regarding Cov-2 is that it continues to not significantly mutate, so that people’s natural immunity following the first infection does not expire soon, and so, it will apply to repeated exposures to the virus in the coming months and years.

It is known how many people were tested positive for the virus (confirmed cases), but not how many people are infected: too many people have not been tested. It is not known how many people were infected but are not showing any symptoms. It is not known how contagious are people who are asymptomatic.

The virus’s Ro, that measures how contagious it is (the average number of people that one person infects), is different in various populations within a country and in various countries, depending on various factors such as population density (cities vs. the countryside), household structure (single households vs multigenerational households, community living like nursing homes, etc.), social distancing practices, etc.

An article regarding the U.S, reads that given an average Ro of 2- 2.5,  at least 50% of the U.S. population needs to be immune to the virus for the herd protection to apply, and that only 2-3% of Americans recovered from Covid-19 so far, which is far from the 50% required.

Therefore, it is not expected that herd immunity will be reached before massive vaccination is available, hopefully in 2022. In the meantime, what is expected is that cities and states “open things up slowly, test widely to track the virus’s spread, and impose lockdowns again before new waves of infection grow too large.. social distancing measures could be necessary on and off through 2022, according to an analysis from infectious-disease researchers at Harvard, published Tuesday in the journal Science”.

Elizabeth Halloran is a biostatistician at Fred Hutchinson Cancer Research Center and the University of Washington) “says we can make it until a vaccine is available by quickly identifying and isolating infected people, tracking down who they may have infected, and providing enough protective equipment for frontline workers… If we can mitigate or suppress infection, we would be living with some infection and probably some deaths, but it wouldn’t be overwhelming. The infection would sort of limp along in the population because we wouldn’t necessarily stamp it  out, but it wouldn’t travel very fast.. That’s a different concept than herd immunity.”

* The above info is from the following sources: ocregister. com/2020/04/16/ c0ronavirus-gove,   technologyreview. com/ 2020/04/14.. why-simply-waiting-for-herd-immunity-to-covid-19-isnt-an-option,  and the source for the quotes above is: businessinsider. com/ us-covid-19-herd-immunity-2020.

anita

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