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Updated: Jul 4, 2022

General idea about a vaccine -

COVID related -


What is a vaccine?

A vaccine is a cocktail consisting of biological and inorganic/organic molecules that provide immunity from a set of or a specific disease.

This cocktail consists -

  1. Antigen - Main element that generates immunity.

  2. Adjuvant – An inorganic/organic molecule working as an assistant.

  3. Preservatives - maintain shelf life

  4. Stabilizers - maintain the active state

What are antigens?

Antigens are biomolecules (nucleic acids/proteins) in active/inactive forms, derived from the pathogen against which immunity is generated.

The conventional vaccines contain either a portion derived from the pathogen or an inactivated complete pathogen. Some of these vaccines can also have antigens derived from similar organisms from the same or different families.

A few examples of conventional covid vaccines are listed below -





Manufacturer (in India)



Adenovirus vector


Serum Institute of India



Inactivated virus

Bharat Biotech and ICMR-NIV

Bharat Biotech



Adenovirus vector

Janssen Pharmaceuticals Companies of Johnson & Johnson

Janssen Pharmaceuticals Companies of Johnson & Johnson



Receptor binding domain of SARS-CoV-2

Biological E Limited

Biological E Limited



Engineered Baculovirus containing a modified version of SARS-CoV-2 spike protein


Serum Insititute of India

With advancements in vaccine development and production, RNA and DNA derived from the pathogen are now used as antigens. This direct introduction relies on the expression of the antigen (protein) within the injected tissue/cell through the process of transcription and translation. This stimulates a broader range of immune responses involving B and T cells.

As it's said, hard times bring the best in us; similarly, this pandemic forced us to innovate and take risks. While RNA and DNA-based vaccines were in theory and trials for around a decade, this pandemic accelerated our efforts giving us the first commercial mRNA and DNA-based vaccine.

  • mRNA-1273:- Developed by Moderna, a US-based pharmaceutical company. The antigen here is a modified mRNA of SARS-CoV-2 spike glycoprotein. This vaccine will be manufactured in India by Cipla.

  • ZyCoV-D:- Developed by Indian pharmaceutical company Cadila healthcare. The antigen here is gene encoding spike protein of SARS-CoV-2 and delivered using a plasmid vector.

How does vaccine generate immunity?

Antigens present in vaccines activate the immune system of our body. Antigen first binds to Dendritic cells that are antigen-presenting cells. Later in the lymph node, MHC molecules present peptides from the antigen to T cells through their T cell receptors, stimulating B cells' development for antibody production. Antigen presentation to T cells induces proliferation of T cells into effector T cells and memory T cells. Further, this also stimulates the maturation of B cells into memory B cells and plasma cells. Effector T cells are ready to encounter the pathogen, whereas memory B and T cells register the antigen present in the vaccine and can get activated on reinfection. Short-lived plasma cells secrete the antibody and increase the antibody titer, whereas long-lived plasma cells travel to bone marrow and continue producing the antibody for longer.

Why are people getting covid infections even after vaccination?

The purpose of a vaccine is to provide immunity against the infection rather than preventing entry of the pathogen within our system (body). In simpler words, vaccines prepare the weapons, i.e., antibodies and memory cells required to fight against the infection. This will lessen the severity of the disease rather than prevent the onset of the infection. Therefore, one can get re-infected by covid or any other pathogen against which one has been vaccinated with less severity.

How is immunity developed post-vaccination different from that developed post-infection?

Several scientific studies report that the immunity developed post-COVID infection is more specific. That means the antibodies developed due to the infection can only target the infecting strain of COVID. For instance, an individual infected by an omicron variant will then develop antibodies targeting only the omicron strain of COVID. However, that individual will still be susceptible to severe covid infection by other strains such as delta, gamma, etc. Whereas, vaccines provide a broader range of immunity by building antibodies that can target most of the strains or variants.

Why is booster dose necessary?

With time the immune response developed post-vaccination gets diluted due to loss of memory cells and antibodies. Therefore booster dose allows recirculation of active antibodies within our system and updates memory cells, thereby boosting our immunity.

Edited by - Ms. Debashmita Sarkar

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