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?

A vaccine works on the basic immunological principle of controlled priming: exposing the body to a harmless form or component of a pathogen (an antigen) so the immune system learns to recognise and neutralise it without causing disease.

How vaccines work

• The injected antigen is detected by the innate immune system and presented to lymphocytes.
• B-cells produce antibodies against the antigen, while T-cells provide cellular immunity.
• Crucially, the body forms memory cells. On later exposure to the real pathogen, these mount a rapid, strong secondary response, preventing or limiting infection.
• Widespread vaccination also builds herd immunity, protecting those who cannot be vaccinated.

Approaches adopted by Indian manufacturers

• Inactivated whole-virus (Covaxin): Bharat Biotech, with ICMR-NIV, grew and chemically inactivated SARS-CoV-2, using an adjuvant to boost the response — a classical, indigenous platform.
• Viral-vector (Covishield): Serum Institute of India manufactured the Oxford-AstraZeneca vaccine, using a modified chimpanzee adenovirus carrying the spike-protein gene.
• Protein-subunit (Corbevax): Biological E used the receptor-binding domain of the spike protein with an adjuvant.
• DNA-based (ZyCoV-D): Zydus Cadila developed the world's first approved plasmid-DNA vaccine, needle-free and administered intradermally.

Conclusion

India's use of inactivated, viral-vector, protein-subunit and DNA platforms showcased its vaccine-manufacturing depth, enabled the vast Covid-19 vaccination drive, and reinforced its role as the "pharmacy of the world."