Whats is Herd Immunity | How does it Work?

A network or group of sound individuals. An irresistible disease would go effectively through the network from individual to individual.

Vaccines train our immune system to protect us from diseases it hasn’t come into contact with before.

For example The MMR vaccine provides immunity against

  1. Measles
  2. Mumps
  3. Rubella

During an outbreak of a disease vaccinated people will remain healthy. As they won’t become infected they won’t pass the disease on.

When most of the community is vaccinated it makes it difficult for the disease to spread within the community. This is called Herd immunity.

If more people within the community remain unvaccinated herd immunity can break down allowing the disease to spread more easily.

Herd immunity is especially important for individuals in the community who can’t be vaccinated.

For example if they’re too old too young or have weak immune systems.

 

How Herd Immunity Works?

The term herd immunity is often heard in the news in regard to vaccinations. At an individual level, vaccines prevent people from getting a specific disease, be it measles or polio.

But at the population level, it is not always necessary for everyone in the population to have immunity to a disease to prevent a widespread epidemic in the population.

This is because there is a certain threshold of immunity in a population that prevents a disease from becoming widespread.

Let’s take a closer look with a Herd Immunity example at how this happens.

In any given population there are 3 groups of people with regard to any specific disease-causing organism.

specific disease-causing organism

In cases of communicable diseases an epidemic begins when one person is infected and becomes contagious or is capable of spreading the disease to others. First Scenario.

First Scenario

For example, each year we experience an epidemic of chickenpox and influenza in the general population.

The infected individual comes into contact with an individual who does not have the disease but is not immune to it either.

An exposure event can be breathing the air in a room where someone is sneezing or coughing or it can be direct contact with the infected person.

Either way an exposure occurs and the susceptible person now has the organism in their system.

At this stage the exposed individual may fight off the disease and become immune without showing symptoms.

Exposure does not automatically mean infection with symptoms. However, most infectious diseases are categorized as diseases because they’re quite good at making us sick and causing illness.

We’ll assume this is the case here with the chickenpox exposure. At this stage the second person becomes sick and infectious.

This person can go on to expose another healthy but susceptible person to the infectious organism. That person then becomes sick and infectious.

Then another susceptible individual becomes exposed and then becomes sick themselves. and the process continues on and on.

infection from one person to another diagram

In this way the spread of the disease-causing organism quickly moves through the population creating a chain of infection that continues unabated.

So, if the point of medical intervention is to prevent such chains of infection. How to Break the infection chain?

Second Scenario

Lets’ start again with an initial sick individual. This person is able to pass the disease-causing organism on to a new susceptible person.

The second individual is now sick and capable of passing the disease on to another. However, in this case, the next individual in the chain is already immune to this particular infectious organism.

Infection chain break diagram

In this example, the person had either already had chicken pox, gotten better and is now immune or they were vaccinated and have become immune.

At this point the chain of infection is broken and no more people are exposed or become sick. In other words, no epidemic occurs.

Third Scenario

Now let’s consider a more realistic scenario and see how diseases are transmitted through a population.

Here we have one sick individual who comes into contact with 4 other people who are not immune to this infectious organism.

After all, this is a more likely scenario, someone who is sick can come into contact with many people. We’ll only consider 4 here though in reality it could be dozens of people, say in a classroom.

One sick child can expose 20 other students in elementary school or perhaps a couple hundred in a college lecture hall.

Of the 4 people exposed, each of them becomes infected, shows symptoms and can now expose another 4 people to the disease.

This next group of people becomes sick and goes on to infect another 4 individuals who then go on to infect another 4 people.

In the end, since everyone in the population is susceptible, everyone comes down with the disease.

how diseases are transmitted through a population

This may not be a problem for chickenpox where the incidence of death is extremely low -but say this is the pattern for Ebola, where more than 80 or 90% will die without extensive medical intervention.

High mortality rates present a serious problem when everyone exposed can become sick.

Fourth Scenario

So what happens when we start to account for a small number of people in the population who happen to already be immune to the infectious organism?

In this case there are only a few immune individuals in the population. Will this be enough to protect the population as a whole?

The initial patient exposes 4 people to the disease, however, one of these 4 is immune.

That means that all of the people down the line who otherwise would have been exposed to the infectious organism, now will not be exposed.

However, there are 3 infected individuals who can now spread the disease onward.

Of the 3 sick individuals, only two are able to pass on the infectious organism because one of the next 3 in the chain happens to be immune.

The disease is blocked from spreading. Now there are only 2 sick individuals to pass the disease on to the next individuals in the chain.

However, of the two sick individuals, only one happens to pass along the disease because the other has exposed an immune individual who cannot become infectious and spread the disease further.

Now there is only one person left who is infectious and who manages to spread the illness to one last person.

Now there are only 2 sick individuals to pass the disease on to the next individuals in the chain.

However, of the two sick individuals, only one happens to pass along the disease because the other has exposed an immune individual who cannot become infectious and spread the disease further.

Now there is only one person left who is infectious and who manages to spread the illness to one last person.

Now the outbreak has run its course but look at the results. Six individuals were prevented from even coming into contact with the illness thanks to the few who were immune. They broke the chain of infection.

Infection chain break

However, seven individuals did come down with the illness, more than were protected. This would be considered an epidemic outbreak. If this illness were a fatal one, almost half of the population would have died.

Fifth Scenario

Let’s see what happens in a case where we have a mere 50% immunity in the population – a level considered far below effective levels of herd immunity.

We begin with our first infected individual who exposes 4 other people. In this case, half of the individuals exposed to the infectious agent are already immune.

However the other half do become sick and infectious. Of these two only one is able to pass the infection on to a susceptible individual as the other exposed individual is immune.

At this stage the remaining infected patient has a 50/50 chance of exposing the infectious agent to a susceptible individual.

In this example the next person happens to be immune so the chain of infection is broken and the epidemic is thwarted.

Even at 50% herd immunity, only 3 members of the population become sick. There are many more individuals who were capable of becoming infected.

There are many more individuals who were capable of becoming infected but since they are surrounded by others who are immune, they never became exposed, thus they never became ill.

Still, having about 20% of your population dying off would be considered unacceptable. Ideally, it is most effective to have herd immunity in the 85 to 95% range.

When this level of immunity is achieved the odds are pretty low of an infected individual being able to expose a susceptible person to the disease.

Even if this does happen, the odds are again low of that second person being able to find and infect a third one.

herd immunity explaination

Even in situations where a few cases appear the disease cannot spread to the population as a whole and cause a massive outbreak when herd immunity is sufficiently high.

Even if this does happen, the odds are again low of that second person being able to find and infect a third one.

Even in situations where a few cases appear the disease cannot spread to the population as a whole and cause a massive outbreak when herd immunity is sufficiently high.

Importance of Herd Immunity

Importance of Herd Immunity

In this Part we will discuss

  • Why the flu vaccination protects more than just yourself?
  • How early on the flu can be passed to others?
  • How the flu can affect high-risk individuals?

So when you get vaccinated for the flu we commonly think about doing it for ourselves.

We are trying to protect ourselves from becoming sick losing work time or just having to deal with the burden of being ill ourselves.

But really the the thing that you can do for your community and help others is actually to vaccinate yourself.

There are people who cannot receive the flu vaccine because either.

They’re too young for example children under the age of six months or too young to receive the flu vaccine.

There are people who have allergies to the vaccine and can’t receive it.

There are others who for other health conditions may even get the flu vaccine and herd immunity but are still at very high risk for contracting it.

So when you’re able to vaccinate yourself you do help prevent the spread of flu to this population as well.

Some people will think well if I don’t get the flu vaccine I know I could get sick and pass it to others but if I feel like I’m getting ill I’ll just avoid spreading it to other people by maybe quarantine II myself at home.

I know my mother has COPD or asthma I know she could get very sick from the flu so I just won’t visit her if I’m not feeling well.

The problem with that theory is that the flu virus can be contagious in people one to two days before they ever feel ill.

Her Immunity theory

So unfortunately although you may have the best intentions you could still be spreading this virus to people even though you’re not feeling sick yourself.

So again if you’re to vaccinate yourself not only do you help yourself from preventing get the flu and protect these vulnerable populations but you also help inadvertently spreading it to people.

Unfortunately 20 to 50 thousand Americans die every year of the flu.

Who is at high Risk?

  • Unfortunately these high risk populations such as
  • Children
  • The elderly Peoples

Adults with chronic conditions specifically asthma or COPD and those that develop unfortunate complications related to the flu like pneumonia are at the highest risk of dying.

Again to protect this population and yourself from developing complications of the flu or increased risk of mortality from the flu.

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