Immunisation and Vaccines


Immunisation refers to the process of both getting the vaccine and becoming immune to the disease following vaccination. Immunisation is an important component of primary health care. Currently it prevents 2 to 3 million deaths every year from vaccine preventable diseases.

What is vaccination?

The both terms vaccination and immunisation often used interchangeably. The act of administrating the vaccine into the body is termed as vaccination. Not only do vaccinations prevent sickness and death associated with infectious diseases, they also help limit the spread of antibiotic resistance and reduce preventable illness and deaths.

Why is vaccination important?

Vaccination is a simple, safe, and effective way of protecting people against harmful diseases, before they come into contact with them. About 20 diseases, such as polio, diphtheria, pertussis, tetanus, and measles can be prevented by vaccination. When we get vaccinated, we aren’t just protecting ourselves, but also those around us. Some people like those who are advised not to get certain vaccines, depend on the rest of us to get vaccinated and help reduce the spread of disease. 


Vaccines are biological preparations, when administered stimulate a person’s immune system to produce antibodies against infectious agent in a similar way as it does when it’s exposed to a disease. If we are exposed to such organism/germ in the future, our immune system responds and can quickly destroy it.

Vaccines do not cause the disease as they contain only killed or weakened forms or inactive parts of a particular organism (antigen) that triggers an immune response within the body.

Some vaccines require multiple doses, given weeks or months apart for the production of long-lived antibodies and development of memory cells.

Vaccines are administered through various routs. Most vaccines are given by injections but some are given orally (by mouth) or sprayed into the nose.

How do vaccines wok ?

Vaccines train the immune system to create antibodies in a similar way as exposed to a disease organism. So when vaccine is given, immune system responds in the following way:
  • It recognizes the invading organism, such as the virus or bacteria (antigen) present in the vaccine,
  • produces antibodies to fight organism,
  • remembers the organism and
  • in the future, if a person is exposed to such organism/germ, the immune system responds and can quickly destroy it.

(Antigens are the components /fragments from organisms such as the virus or bacteria or their toxins).

(Antibodies are proteins produced by the body to neutralize or destroy toxins or disease-carrying organisms. Antibodies are specific for particular diseases.)

What are different types of vaccines?

The different types of vaccines are-

Whole- pathogen vaccines consist of entire pathogens that have been killed or weakened so that they cannot cause disease but produce immunity. Whole- pathogen vaccines can be of two types-
  • Inactivated vaccines contain killed pathogens (by using chemicals, heat or radiation). Some of the examples are –
    • Hepatitis A 
    • Rabies
    • Inactivated polio virus
  • Live-attenuated vaccines contain weakened (or attenuated) form of the germ that causes a disease. Vaccines like this may not be suitable for people with compromised immune systems. Following are some of the live attenuated vaccines-
    • BCG (tuberculosis)
    • Oral polio vaccine (OPV)
    • Measles, Measles mumps rubella (MMR)
    • Rotavirus
    • Yellow fever
Subunit Vaccines: Instead of the entire pathogen, subunit vaccines include only the components or the very specific parts (the subunits) of a virus or bacterium that best stimulate the immune system. There are several types of subunit vaccines, such as-protein subunit vaccines, polysaccharide vaccines and conjugate subunit vaccines. Some of the subunit vaccines are- 
  • Haemophilius influenzae type B (HIB)
  • Pneumococcal (PCV)
  • Hepatitis B (HepB)
  • Human papilloma virus

Toxoid vaccines- These vaccines contain a toxin (harmful product) made by the germ that causes a disease. They create immunity to the part of the germ that causes a disease instead of the germ itself. The antigens in toxoid vaccines are chemically inactivated toxins, known as toxoids. Some of the toxoid vaccine are-

  • Diphtheria toxoid
  • Tetanus Toxoid

Nucleic acid vaccines: Nucleic acid vaccines use genetic material from a disease-causing virus or bacterium (a pathogen) to stimulate protective immunity against it. The genetic material could be DNA or RNA. This is a relatively new technology though several DNA vaccines are licensed for animal use, including a horse vaccine against West Nile virus.

Viral vector-based vaccines: Some vaccines use a harmless virus or bacterium as a vector, or carrier, to introduce genetic material into cells, triggering protective immunity against it. An example of a viral vector vaccine is the rVSV-ZEBOV vaccine against Ebola.

Two types of viral vector-based vaccines are there.

  • Non-replicating vector vaccines (cannot replicate within cells)
  • Replicating vector vaccines (can replicate within cells)

Immunity is the ability of the body to resist a specific infectious disease. It develops when the cells of immune system are exposed to harmful organism/pathogen, such as a virus or bacterium they create antibodies and successfully fight it off.

(A pathogen is a bacterium, virus, parasite or fungus that can cause disease within the body.)

Immunity can be innate immunity or acquired immunity.

Innate immunity: The protective mechanisms with which we are born such as cilia, skin, mucous membranes of the throat and gut act as innate immunity. However, these responses are not specific to a particular pathogenic agent and innate immune system does not have memory.

Adaptive or acquired immunity: If the pathogen manages to overcome the innate immune system, adaptive or acquired immunity starts to develop. It can develop through infection with the actual disease and called as natural immunity, or through the introduction of a killed or weakened form of the disease-causing pathogen, or fragment of it, as in the case of immunisation and it is called vaccine-induced immunity.

Adaptive or acquired immunity persists as even after the pathogen is eliminated from the body, specialised ‘memory cells’ are there to act against that pathogen if body encounters it again.

There are two types of acquired immunity-Active and passive:

  • Active immunity is produced when after an exposure to a disease organism; the immune system triggers the production of antibodies to that disease. Exposure to disease causing organism can occur either by natural infection (natural immunity) or through introduction of vaccine (vaccine-induced immunity). In both ways the immune system will recognize the disease causing organism in future and try to destroy it by producing antibodies. Active immunity is long lasting and sometimes lifelong.
  • Passive Immunity is achieved when a person gets antibodies to a particular disease rather than producing them through his or her own immune system. Such as-  
  • A baby receives antibodies from the mother through the placenta before birth and in breast milk after birth. This passive immunity protects the baby from some infections during the early years of their life.
  • A person can also get passive immunity through antibody-containing blood products such as immune globulin, which may be given when immediate protection from a specific disease is needed.

The major advantage of passive immunity is that it gives immediate protection, whereas active immunity takes time (usually several weeks) to develop and provide protection. However, passive immunity lasts only for a few weeks or months.

What is herd immunity?

Herd immunity: It is also known as 'population immunity’. Herd immunity is the indirect protection from an infectious disease that occurs when a population is immune either through vaccination or immunity developed through previous infection. If enough people are vaccinated it becomes harder for the disease to spread to those who cannot have vaccine, people who are ill or have weakened immune system.

The percentage of people who need to be immune in order to achieve herd immunity varies with each disease. For example, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated.

Adverse events following immunization (AEFI)

An adverse event following immunization (AEFI) is any untoward medical occurrence which follows immunization and which does not necessarily have a causal relationship with the usage of the vaccine. The adverse event may be any unfavourable or unintended sign, abnormal laboratory finding, symptom or disease.

AEFIs are grouped into five categories by WHO-

  • Vaccine product-related reaction- An AEFI that is caused by a vaccine due to one or more of the inherent properties of the vaccine product. Example- limb swelling after DPT vaccination.
  • Vaccine quality defect-related reaction- An AEFI that is caused by a vaccine due to one or more quality defects of the vaccine. Example: paralytic polio after inactivated polio vaccine due to improper inactivation of lot of inactivated polio vaccine   
  • Immunization error-related reaction: An AEFI that is caused by inappropriate vaccine handling, prescribing or administration and thus it is preventable. Example: transmission of infection by contaminated multidose vial.
  • Immunization anxiety-related reaction: An AEFI arising from anxiety about the immunization. Example: Vasovagal syncope in an adolescent during/following vaccination.
  • Coincidental event: Coincidental events occur after a vaccination has been given but are not caused by the vaccine or its administration.

*For purposes of reporting, AEFIs can be classified as minor, severe and serious

Minor AEFI are common, self-limiting reactions, e.g. pain, swelling at injection site, fever, irritability, malaise etc.

Severe AEFI can be disabling and rarely life threatening and do not lead to long-term problems. Examples of severe reactions include non-hospitalized cases of anaphylaxis that has recovered and high fever (>102 degree F), etc.

Serious AEFI includes death, requires inpatient hospitalization, results in persistent or significant disability, AEFI cluster or evokes significant parental/ community concern.



COVID-19 Vaccines

While strong measures have been taken by India to contain the spread of COVID-19 through better diagnostics and treatment strategy, the successful introduction of the COVID-19 vaccine will further help in controlling the disease by enhancing immunity and containing the disease spread. Globally, various scientific techniques are being used for safe and effective COVID-19 vaccine such as:

Virus vaccines: These vaccines use the virus itself in a weakened or inactivated form. There are two types of virus vaccines against coronavirus, weakened virus and inactivated virus vaccines.

Viral-vector vaccines: In the development of these vaccines, a virus (such as adenovirus or measles) is used as vector or carrier and is genetically engineered to produce coronavirus proteins in the body, triggering protective immunity against it. The two types of viral-vector vaccines under development are replicating viral vector (can replicate within cells) and non-replicating viral vector (cannot replicate within cells).

Nucleic-acid vaccines: Nucleic acid vaccines use genetic material from a disease-causing virus or bacterium (a pathogen) to stimulate protective immunity against it. The genetic material could be DNA or RNA.

Protein subunit vaccines: Protein subunit vaccines use fragments of protein from the disease-causing virus to trigger protective immunity against it. The two types of protein-based vaccines being developed against the coronavirus are the protein subunit vaccines and virus-like particle vaccines.

In India, two types of vaccines Covishield and Covaxine have been rolled out for vaccination. Covishield is based on viral vector technology and Covaxine is a whole virion inactivated corona virus vaccine. Two doses of vaccine are given at four weeks apart.

The fraction of the population that must be vaccinated against COVID-19 to induce herd immunity is not known. This will vary according to the community, the vaccine, the populations prioritized for vaccination, and other factors.  

Achieving herd immunity with safe and effective vaccines makes diseases rarer and saves lives. 

Know more about COVID-19 Vaccination in India:

Audio visual on Dr Guleria, Director AIIMS, Delhi sharing FAQs on COVID-19 Vaccine rollout

Frequently Asked Questions on COVID-19 Vaccine

COVID-19 Vaccine Communication Strategy

COVID-19 Vaccines Operational Guidelines Updated as on 28 December 2020








  • PUBLISHED DATE : Jan 11, 2016
  • PUBLISHED BY : Zahid
  • LAST UPDATED ON : Mar 15, 2021


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