Herd immunity

Herd immunity

Many of you already know about immunity. Many of you know what is herd. Here we will see what is herd immunity and how does it work? Herd immunity is a basic concept in epidemiology. Epidemiology though is utterly boring and difficult we will make it very easy and interesting for you.

To give you a background idea we will first start from the basic terms going into details about further terms.

With some of these concepts like epidemiology and infectious disease you will further get idea about Herd immunity. This details description will help even a new person to understand the details of the topic. So enjoy it as we further dig into it.

What is epidemiology?

Epidemiology is the field that studies about diseases. Like medicine studies about disease and how it affects individual epidemiology studies diseases and how it affects the society. Clinician diagnoses and treats a disease in a patient and epidemiologist diagnoses the root cause of disease and helps to treat and prevent that disease or group of diseases in a population. So in simple terms epidemiologists are doctors of a population. 

Epidemiology helps to detect the disease in community defining it then studying many demographic factors involved in disease it helps to find out the cause of disease and also to find out the treatment on a larger scale. Once the cause of disease is known with the epidemiological data it is very easy to conquer the fight against disease as it can be prevented and treated scientifically.

So if clinician helps to come to your normal life from a ill help, epidemiology helps the entire community to function better or normal helping in fight against a disease. Epidemiology treats ill community.

What is infectious disease?

This is second important concept in understanding the herd immunity. You may have read and heard on media about infectious disease. So, what is the meaning of the term infectious disease? Disease is a state which is abnormal for your body, or mind. The cause of disease can be an infection, injury or poor unhealthy lifestyle.

Infectious disease is a disease which is caused by the infection. To get an infectious disease infection is the phenomenon that is must to occur. So in cause of infectious disease there must be an infecting agent. This infecting agent can be visible like parasite or invisible like bacteria and virus to naked eyes. 

When these infecting agents gains entry into your body they multiply themselves and they utilize your body as their home and food producing the bi-products and bad effects on your body those manifesting as a disease. The symptoms of these diseases can be as trivial as simple fever or may be deadly to cause loss of life. Various infecting agents cause various diseases.

The most needed event infectious disease to thrive in community is spread of disease from one person to other. This event may be person to person contact, sexual activities, through air or through water and food. 

The second most important event in this phenomenon or process is the susceptible individual. The individual after getting infected may show symptoms or may not but he/she may unknowingly harbor and spread the infection.

What is immunity?

Before understanding the term heard immunity it is very important for you to know the term immunity. 

Immunity as many of you may know is a protective mechanism of human body that protects you from the disease. Everyone has immune mechanism that functions as immunity.

We have discussed about infectious disease and its spread in above paragraph. If there is infectious disease caused by infecting agent like bacteria, virus, parasite or fungus or whatever, nature has given us mechanism to fight against all those attacking us. Right from birth canal we are attacked by many of these deadly attackers. We survive because we over power them all. This is the role of our immune system also called as immunity. When these invisible agents overpower your immunity you experience or suffer the disease.

How do we get immunity?

Apart from natural immunity AKA innate immunity present in all of us at birth we develop our immunity fighting all these attackers actively acting against them. This is called as adaptive immunity.

Adaptive immunity is developed in process of infection and disease when infection occurs. It may stay with you lifelong or for some months depending on infecting agent.

Not only by getting disease we also get adaptive immunity by vaccination. Vaccines are the tools that help us to develop our immunity without us suffering from the disease. The sole purpose of the vaccine is providing immunity against disease without actually getting that disease. Vaccines are killed infecting agents or live but weakened infecting agents those when ingested or injected recognized by your immune system and necessary mechanism is developed against those diseases.

What is herd immunity?

Herd immunity is obviously the crux of this blog post, lol. As you know herd means group of people and we have explained about immunity in above discussion, now we will discuss about herd immunity.

Like we discussed that for a spread of infecting disease infectious agent and susceptible individuals to infection should be there. If there is infectious agent but there are no susceptible individual obviously the disease will not spread. 

The same is true in case if very few people in community are susceptible but all others are not susceptible to disease as they can fight the disease effectively. If you are susceptible but all around you are immune that is they cannot get infection, there is very less possibility that you will get infection unless you come in contact with other susceptible infected person.

So in this case you are protected from infection not because of your immunity but because there are less chance that you will get infected in a herd.

So we hope this explains the fact about herd immunity.

How to develop herd immunity?

Being all the detailed background explained above now knowing the benefit of herd immunity, you will be definitely interested in knowing how to develop heard immunity.

Well there are two ways to develop herd immunity. One is by getting disease and other is vaccinating the large portion of the population.

Obviously if you allow the first option which may be possible for mild disease with low or no mortality, for deadly and sever disease this won’t be acceptable option. In case of sever disease while spreading in community it will sicken the large population and death and morbidity toll will be unacceptably high. Many families will experience loss of life. Only the fit who can survive the disease will be left with developed immunity. So, how you feel experiencing survival of the fittest phenomenon? Obviously no one would like it.

To bypass this fatality and loss of life we have other viable option. That is getting immunity by vaccination.

When a vaccine is given, it contains antigens of disease causing agent. This when injected or ingested the human immunity recognizes the antigens and it fights against those. The mechanism to identify those quickly and start immune response is developed. When the real infection by real disease causing agent occurs the immunity fights vigorously and you are protected from the disease. Thus all the sufferings and deaths from the disease are avoided and yet the immunity is developed. 

When the vaccination is done in a population in a large proportion, many of the population will be immune to disease. Thus those who have not immunized or got disease and yet susceptible are automatically protected as the do not come in contact with infected person.

This is the function of herd immunity.

What are the applications of herd immunity?

Vaccination can provide herd immunity and we have evidence that when herd immunity develops there is the possibility of extinction of diseases like polio and smallpox.

n quarantine and isolation of suspected infection or infected person is kept isolated and surrounded by already immune persons stopping the spread of the disease to susceptible.

The effectiveness and likely impact of vaccination on disease epidemiology depend on herd immunity in the population. The vaccine will be useless and needless if there is already herd immunity to a certain level.

Where does herd immunity not work?

Herd immunity will not work for diseases which will not spread person to person.

Herd immunity will not work where a disease-causing agent can also thrive in environment like in soil and water independently eg. Clostridium tetanus spore can exist in soil for an extended amount of time. Here the person is not protected unless he/she is individually vaccinated.

Herd immunity will not work in case the infectious agent also thrives in other animals and birds.

Herd immunity will not work for non-infectious diseases like cardiovascular disease and diabetes mellitus.

How long does it take to develop herd immunity?

Depends on multiple factors. Some of the simple words are as follows:

  • How many people a person with the disease can infect?
  • How many people in a population are susceptible to infection
  • How many people in the population are already immune to the disease
  • How many people in the population got infected?
  • Whether all those getting the disease are surviving or dying of disease in a short span?
  • Whether people develop immunity to disease after infection or not.
  • How effective is the vaccine?
  • How many people in the population are given vaccines?



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