What is the scientific explanation for herd immunity?

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Summary:

Is herd immunity real? If so, what is the physiological explanation for herd immunity?

Main Question or Discussion Point

We are hearing more of herd immunity particularly in the wake of the recent SARS-CoV2.

What is herd immunity? Is it real? Or is it a mere statistical reality?

How come somebody who is not vaccinated would be protected because more of somebody else is vaccinated? Logically I am unable to fathom.

Do we have any scientific/physiological/immunological explanation for the development of herd immunity? Is there any research papers that delved into the scientific mechanism behind the herd immunity rather than a statistical model.
 

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  • #2
phyzguy
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A simple explanation is as follows. When modeling the spread of infectious diseases, there is a parameter called R, which is the number of other people an infected person will infect. If this number is greater than 1.0, the number of cases will grow exponentially. If it is less than 1.0, the number of cases will shrink and the disease will die out. In a population where nobody is immune, the R number is referred to as R0. For SARS-COV-2 in a population where nobody is immune, R0 appears to be around 3. As more people become immune, R will drop, because an infected person will be exposed to fewer people who can catch the disease. Again for SARS-COV-2, if about 70% of the population is immune, then R will drop(approximately) to R0 * (1-0.7) = 3.0*0.3 = 0.9. So a person who is infected will infect fewer people, and the chain of infection will die out. The larger R0 is, the higher a percentage needs to be immune in order to achieve herd immunity. For example, for measles, the base R0 is about 14. So you need over 90% of the poulation to be immune (vaccinated) to achieve herd immunity. Does this make sense?
 
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Does this make sense? Yes, statistically. Thanks anyway for your prompt reply.

If R0 is less than 1.0, it infects fewer people. But infects - it is. Isn't it?

OK, it won't grow to a pandemic or epidemic level, but still, it is infectious. People who are already infected will have their own antibodies to protect them. But what about those who haven't the antibodies? They are still susceptible to infection. Isn't it?

So herd immunity gives a breathing space for Governments and authorities. Individuals are still vulnerable.

Am I right in my presumption?
 
  • #4
Vanadium 50
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If Bob is vaccinated, it can't affect the probability you will be infected if exposed. But it will reduce your overall risk because now you can't contract it from Bob.
 
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  • #5
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Thank you. I see the point now.

But I am still wary physiologically. Why won't the already infected persons or vaccinated persons - 70% - can't be considered as the silent carriers (asymptomatic)?

Vaccinated Bob, still as a carrier can always infect, isn't it? Perhaps they may be less virulent.
 
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  • #6
Ygggdrasil
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A simple explanation is as follows. When modeling the spread of infectious diseases, there is a parameter called R0, which is the number of other people an infected person will infect. If this number is greater than 1.0, the number of cases will grow exponentially. If it is less than 1.0, the number of cases will shrink and the disease will die out. For SARS-COV-2 in a population where nobody is immune, this number appears to be around 3. As more people become immune, R0 will drop, because an infected person will be exposed to fewer people who can catch the disease. Again for SARS-COV-2, if about 70% of the population is immune, then R0 will drop(approximately) to 3.0*0.3 = 0.9. So a person who is infected will infect fewer people, and the chain of infection will die out. The larger R0 is, the higher a percentage needs to be immune in order to achieve herd immunity. For example, for measles, the base R0 is about 14. So you need over 90% of the poulation to be immune (vaccinated) to achieve herd immunity. Does this make sense?
Technically, basic reproductive number (R0) is defined as the expected number of individuals a single individual will infect in a population where everyone is succeptible. As a greater fraction of the population becomes immune to the disease, the effective reproductive number (R) decreases, but R0 does not change.

Thank you. I see the point now.

But I am still wary physiologically. Why won't the already infected persons or vaccinated persons - 70% - can't be considered as the silent carriers (asymptomatic)?

Vaccinated Bob, still as a carrier can always infect, isn't it? Perhaps they may be less virulent.
Ideally, vaccination should prevent individuals from becoming infected and spreading the disease. In other words, most vaccinated individuals won't spread the disease because they won't become infected with the disease.
 
  • #7
phyzguy
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Vaccinated Bob, still as a carrier can always infect, isn't it? Perhaps they may be less virulent.
No. Vaccinated Bob is immune to the virus, so the virus cannot grow in him. If a new virus somehow lands in his body, his immune system quickly renders it inactive so it does not multiply. So he does not shed virus particles that can infect others.
 
  • #8
phyzguy
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Technically, basic reproductive number (R0) is defined as the expected number of individuals a single individual will infect in a population where everyone is succeptible. As a greater fraction of the population becomes immune to the disease, the effective reproductive number (R) decreases, but R0 does not change.
Thank you for correcting me. I edited my original post to reflect this.
 
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  • #9
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There are about 100 vaccines or "vaccines" now under development. We don´ t know which of these eventually work and when.
Somebody might succeed in producing a vaccine that successfully protects the vaccinated person from getting seriously ill - that does not completely prevent multiplication of virus in the person, but ensures the vaccinated person gets a mild/asymptomatic form... which is still infectious.
 
  • #10
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There are about 100 vaccines or "vaccines" now under development. We don´ t know which of these eventually work and when.
Somebody might succeed in producing a vaccine that successfully protects the vaccinated person from getting seriously ill - that does not completely prevent multiplication of virus in the person, but ensures the vaccinated person gets a mild/asymptomatic form... which is still infectious.
In the ideal case, a vaccine completely prevents multiplication of the virus in the person. Vaccinated individuals, when exposed to the disease, should not become infected with the disease, will not contract disease symptoms, and will not pass the virus onto others.
 
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Somebody might succeed in producing a vaccine that successfully protects the vaccinated person from getting seriously ill - that does not completely prevent multiplication of virus in the person, but ensures the vaccinated person gets a mild/asymptomatic form... which is still infectious.
Still, if the effective R is low enough, it will not grow into a widespread pandemic: not fast, anyway.

Also, if the vaccine turns any later infection into a cold, then it is already fine. Just have at least 9x% of the population get the vaccine, and it'll became just another cold.

If R0 is less than 1.0, it infects fewer people. But infects - it is. Isn't it?
If there are enough infected when R falls below 1 then it will take time for the pandemic to ring down. But as long as R stays below 1 it can't develop into a pandemic - while sporadic cases might happen.
 
  • #12
BillTre
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Here is a post I made on immunity in another thread:
just as some background, the term herd (not heard as some have been spelling it) immunity comes from the veterinary field. Many vets are not so involved with treating dogs and cats, but work with farms which have herds of animals (keep closely together). This is a situation in which a disease can rapidly spread through a population.

As @Ygggdrasil said herd immunity is when a large percentage of the susceptible population develops immunity through either exposure to the disease in question or by vaccination.

If enough of the population is immune, then the disease can not effectively spread through the population, because the infectious units (viruses in this case) have a much more difficult time finding a susceptible host (victim).
Herd immunity (by vaccine or by recovery from infection) makes it less likely for a particular virus to find susceptible hosts.
 
  • #13
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The crux of my question was to find out, as an individual, if anyone can rest assured that he is protected because others are protected? Does the herd immunity protect him absolutely?

Let us suppose Pete in a community is neither immunized nor exposed to an infection. But 80% of people around him are either immunized or already exposed to the same infection.

The Government and authorities need not fear a pandemic or an epidemic or force a lockdown.

But what about the individual? Can Pete still get an infection?

I think though the chance is less, still not zero.

1) He could get infected from the carriers from among the remaining 20%.
2) He could get infected from the few silent carriers, from among the 80% also.

Herd immunity protects the govt/authorities from a needless scare. But the remaining individuals are still at risk, though at a significantly lower level.

Am I right?
 
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  • #14
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Herd immunity protects the govt/authorities from a needless scare. But the remaining individuals are still at risk, though at a significantly lower level.

Am I right?
With all parts of that claim in one packet together and with the emphasis, yes.
That's why vaccination usually aims for far higher ratio of immune people in the population than the herd immunity threshold: so the individual who (by any reason) can't get a vaccine got more protection.
Also, this is the reason why anti-vaxxers means increased danger to the rest of the population - to those, who can't get vaccine (by medical reasons, not just by whim).
 
  • #15
BillTre
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This NY Times article discusses herd immunity with respect to the corona virus.

I like the visual representation of herd immunity they presented:
Screen Shot 2020-05-28 at 7.41.28 AM.png

The first case (herd immunity estimate) shows the relative difficulty a virus would have in blindly blundering (as viruses do) onto a new host (with out immunity (either from a vaccine or from immunity built up from a previous infection)) so that it could reproduce.

The greater the number of people in a population with immunity, the better the herd immunity effect.
 
  • #16
Ygggdrasil
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The crux of my question was to find out, as an individual, if anyone can rest assured that he is protected because others are protected? Does the herd immunity protect him absolutely?

Let us suppose Pete in a community is neither immunized nor exposed to an infection. But 80% of people around him are either immunized or already exposed to the same infection.

The Government and authorities need not fear a pandemic or an epidemic or force a lockdown.

But what about the individual? Can Pete still get an infection?

I think though the chance is less, still not zero.

1) He could get infected from the carriers from among the remaining 20%.
2) He could get infected from the few silent carriers, from among the 80% also.

Herd immunity protects the govt/authorities from a needless scare. But the remaining individuals are still at risk, though at a significantly lower level.

Am I right?
Herd immunity will not protect unvaccinated individuals absolutely. Herd immunity certainly reduces the chance of an unvaccinated individual being infected, but it will not eliminate the risk. I will also note that vaccination will not completely eliminate the risk of infection because 1) it takes time for immunity to build up after vaccination, so it is possible to become infected shortly after vaccination and 2) vaccinations are rarely 100% effective. Still, vaccination will greatly increase one's chance of becoming immune to infection. In most cases, one would much rather be a vaccinated individual in a population with herd immunity than an unvaccinated individual.

Why do people keep posting the possibility of silent disease carriers among vaccinated individuals? For an effective vaccine, especially for a viral disease, this possibility should be rare.
 
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  • #17
Laroxe
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Perhaps the important thing to remember is that a virus can't live outside of the body for long and when someone is infected the body will eventually produce antibodies to kill the virus. For the virus to stay alive it must spread to others before the host body kills it, if it can't find a new host that doesn't have antibodies, it will simply die. An extreme example is smallpox in which the level of vaccination made it impossible for the virus to find new hosts, as it only infected humans, the disease no longer exists in nature. The aim is to do the same with polio, so that no one is at risk of infection. Many countries had a sufficiently high level of vaccination against measles, it simply couldn't spread, when cases occurred these were always imported from other areas, in which measles continued to spread. It still couldn't get established in the immune population and so this protected the few people who couldn't be vaccinated as they were never exposed to the virus. As vaccination levels fell in some countries when new cases were imported, there were sufficient numbers of unvaccinated people for the disease to continue to spread.
Herd immunity simply reflects the level of population immunity needed to stop the spread of a disease in the population, even if the vaccine isn't 100% effective it can still work, it simply makes the life cycle of the virus impossible to maintain. Disease like polio and measles only infect humans and it should have been possible by now to have eradicated both, like we did with smallpox. I don't know if we have the antivax movement to thank for preventing these endangered viruses from becoming extinct or whether the measles virus has developed the ability to exploit human stupidity in susceptible populations. :)
 
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  • #18
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So herd immunity gives a breathing space for Governments and authorities.
Governments don't get the disease, people do.
If you are wondering about the policies that reside within a country, then that would be due to culture, budgetary constraints, and of course politics. I am sure that is a different discussion.
 
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Herd immunity for diseases with well established, effective vaccines such as Measles, also protect newborns and others with compromised immune systems that cannot take the vaccine
 
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The rate of infection is proportional to the product of the number of infected (and still contagious) people and the number of susceptible people. The proportionality constant decreases with increasing effectiveness of social distancing. As the susceptible population decreases, the rate of infection decreases until the point where each contagious) person,, on average, can infect less than one other person over the time that he is still contagious (approximately two weeks). Once this happens, not only are fewer people being infected, but also, the number of contagious people also begins decreasing. So the product decreases even more rapidly. This is called herd immunity.
 
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  • #21
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The crux of my question was to find out, as an individual, if anyone can rest assured that he is protected because others are protected? Does the herd immunity protect him absolutely?

The probability of a non-immune person to catch the virus is proportional to number of viruses (or virus density). Individual is absolutely safe when the number of viruses is zero, or when the virus has gone extinct.

Hmm ... the herd immunity is reached when the number of viruses starts decreasing, so it's the peak of virus density, so it's the most dangerous time for non-immune person. Right?


But let's say there is no virus yet in some country, and 95% of people are vaccinated, and the R number is 0.1. Now infected visiting tourist will cause on average 0.111 people to become ill (tourist infects 0.1 people, those 0.1 people infect 0.01 people, and so on). Without the vaccination the tourist would have infected the whole nation. Clearly in this case a non-immune individual is being protected.
 
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The greater the number of people in a population with immunity, the better the herd immunity effect.
Progressively, yes. But in theory in a homogenous population there would be a sharp leap at reproduction factor of unity.
Suppose that in the absence of immunity (whether due to having survived the illness or vaccination) 1 infected person would infect 2,0 people.
In 20 generations, 1 infected alien would infect 2 million people. For 2 generations a week, that would make 2 million infected in 2 and a half months (10 weeks).
Suppose 45 % of population is immune. Then 55 % are still susceptible, meaning 1 infected person would infect 1,1 people.
1 infected alien would infect 2 million people just the same. But in this case taking about 130 generations (65 weeks, or 15 months).
Now suppose 55 % of population is immune. Then 45 % are susceptible, meaning 1 infected person would infect 0,9 people - and 1 infected alien infects a grand total of 9 people, not 2 millions.

How does the "herd immunity" protect a person not himself immune?
Of the people he meets, some are immune. But others, though not immune, have not had an opportunity to meet infectious people. The presence of immune people protects non-immune people by cutting off possible routes for the infection to reach them. Enough immune people, and an infected person infects only a few vulnerable people. If, however, there are too few immune people, the infection will find ways around the immune people, from non-immune people to non-immune people.
 
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  • #23
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Herd Immunity does not guarantee protection to a particular individual, but it decreases the chance of infection for members of the affected population. And it comes down to social contact patterns. Some people visit lots of busy places, some people visit few busy places, some people visit only a friend or two. All of society linked in one big map of contact will have lots of structure to it, but it will also have bottle necks (like one person is the only social link between two subcultures). There's also commercial contact chains (food delivery, restraunts, shopping malls), pubic spaces and shared equipment can transfer disease without humans being there at the same time, etc. So a lot of assumptions about how disease propagates through these contact chains underlies this kind of model and they aren't straightforward.

Often, different handfuls of people are at the center of different inter-connected social networks. If most people didn't vaccinate, disease would spread quickly through these people who act as nodes in these contact networks. Since most people vaccinate, most of the people in these nodes act as a shield, blocking transmission between the different contact points they have with other members of society, and they may contact people who are also at the center of other contact networks (which contain other people at the center of other contact networks). So it's important that all these people are blocking transmission or else infections can propagate through contact networks, spreading wider at each contact point as more members of society are included in the infection chain.
 
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  • #24
atyy
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But I am still wary physiologically. Why won't the already infected persons or vaccinated persons - 70% - can't be considered as the silent carriers (asymptomatic)?
That is a technical possibility. Whether it occurs depends on the disease and the vaccine. It is discussed as a possibility with some whooping cough vaccines.
https://www.sciencedaily.com/releases/2015/06/150624071018.htm
 
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  • #25
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That is a technical possibility. Whether it occurs depends on the disease and the vaccine. It is discussed as a possibility with some whooping cough vaccines.
https://www.sciencedaily.com/releases/2015/06/150624071018.htm
Yes, this is very much an issue that has to be looked at in terms of the disease and the level of immunity. With Covid 19 its an acute disease which is not thought to persist in individuals. Even in acute viral infections there are some immune privileged areas in the body where the virus may persist for longer but for someone to be infectious they have to shed large amounts of viable virus particles into the environment. This means that even when someone is ill with the virus their "infectiousness" varies. In Covid 19 the period in which the infected person presents the greatest risk to others is a few days before they present symptoms to a few days after, on either side of this range the amount of virus being shed, falls off rapidly.
As people suggest there isn't one set answer, there are simply to many factors to consider, but for the virus to continue to spread it has to come into contact with someone who is not immune and then they with other people who are not immune. At a level of immunity in around 70% of the population it is just about impossible to maintain a chain of infection, even if there are people who enter this population with an infection, it is possible some people may become infected but the virus will die out because it can't maintain the chain.
Its unfortunate really that people talk about herd immunity as an all or nothing effect, in reality, the more people that are immune the more difficult it is for the virus to spread. Effectively as more people become immune there is a shift in the R number, transmission becomes more difficult, herd immunity is simply a calculation of the level needed to prevent any sustained transmission.
Unfortunately this is a new disease and a lot of the assumptions are based on the behaviour of other diseases, a fact of life in biology is that organisms in tightly controlled environments have a habit of doing whatever they want. We simply can't really predict where this disease will go, we haven't enough information and we have no information about the relative effectiveness of any vaccine that's developed.
 
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