Several vaccines are under development against SARS-CoV-2. In India, two vaccines have been in the news- COVISHIELD (co-developed by Oxford University and Astra-Zeneca), and COVAXIN (by Bharat Biotech in collaboration with Indian Council for Medical Research (ICMR) and National Institute of Virology (NIV)).

It is important to understand the different types of vaccines and how they work to better understand the recently published article on safety and immunogenicity of COVAXIN.

This article will briefly describe some important concepts and terms in vaccine development.

Vaccination categories:

Active immunization: This type of vaccination stimulates the immune system to produce

• specific antibodies
• cellular immune responses
• or both

and

• prevents/ protects against
• ameliorates
• or eliminates a disease.

Passive immunization: This type of vaccination uses a preparation of antibodies that neutralizes a disease pathogen or binds to a human cellular antigen. Such vaccines are administered before or around the time of known or potential exposure to pathogen/ subject with disease/ infection.

Vaccine formulation: This refers to the vaccine and added components present in the vaccine solution and diluent. The following additives may be present:

• Preservatives or antibiotics (to prevent growth of bacteria in multidose containers)
• Stabilizers (to extend the shelf life of the product)
• Adjuvants (to enhance the immune response)
• Delivery systems (to present the vaccine antigen(s) to appropriate cells of the immune system or preserve/ stabilize the integrity/ conformation of antigen(s) in vivo).

Adjuvants: Adjuvants are substances that, when mixed with vaccine immunogens, potentiate the immune response resulting in the need for either a lesser quantity of antigen or fewer doses, or both.

Immunogenicity: The property of eliciting an immune response

Reactogenicity: Refers to a subset of reactions that occur soon after vaccination and are a physical manifestation of the inflammatory response to vaccination. In the case of a vaccine intended for healthy subjects (particularly infants), minimizing acute reactogenicity is critically important.

Active Vaccines

There are three general categories of active vaccines:

Live vaccine: It is usually a microorganism (virus, for instance) that replicates on its own in the host or infects cells and functions as an immunogen without causing disease.

Inactivated vaccine: It contains an immunogen that cannot replicate in the host because the disease-causing microorganism has been killed with chemicals, heat, or radiation.

Nucleic acid-based vaccine: Usually DNA, this is also incapable of replicating in humans, is taken up by cells and directs the synthesis of the vaccine antigen(s).