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What is the basic principle behind vaccine development? How do vaccines work? What approaches were adopted by the Indian vaccine manufacturers to produce COVID-19 vaccines?

This question was previously asked in
UPSC CSE 2022 (Prelims) CSAT Previous Year Paper (05-June-2022)

The basic principle behind vaccine development is to safely expose the body's immune system to a pathogen, or a part of it, without causing the disease. This exposure acts as a "training exercise" for the immune system.

How Vaccines Work:
When a vaccine introduces a harmless version or component (antigen) of a virus or bacterium, the immune system recognizes it as foreign. It then mounts a primary response, producing specialized cells called antibodies to fight it and memory cells (B-cells and T-cells). These memory cells remain in the body long after the initial threat is gone. If the body is later exposed to the actual, live pathogen, these memory cells quickly recognize it and launch a swift and powerful secondary immune response. They rapidly produce a large number of antibodies to neutralize the pathogen before it can cause a serious illness. In essence, vaccines provide the memory of an infection without the illness itself.

Approaches by Indian Manufacturers for COVID-19:
Indian vaccine manufacturers adopted several proven and innovative approaches:

  1. Inactivated Virus Vaccine: This is a traditional approach used by Bharat Biotech for Covaxin. It involves taking the live SARS-CoV-2 virus, killing (inactivating) it so it cannot replicate, and then injecting it. The immune system recognizes the surface proteins of the dead virus and builds immunity.
  2. Viral Vector Vaccine: This approach was used by the Serum Institute of India to produce Covishield (developed by Oxford-AstraZeneca). It uses a harmless, modified adenovirus (from chimpanzees) as a vector to deliver the genetic code for the coronavirus's spike protein into human cells. Our cells then produce the spike protein, triggering an immune response against it.

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