What are vaccines? 

• Vaccines are one of the most effective tools to prevent diseases.
• A vaccine is “a product that produces immunity from a disease External link  and can be administered through needle injections, by mouth or by aerosol.”
• A vaccine is a substance that resembles the disease-causing agent (also called pathogen) but does not cause the disease.
• It trains the body’s immune system to recognise and kill the pathogen, and creates a memory for the future.
• Vaccines are available to protect us against many life-threatening diseases such as polio, influenza, meningitis, typhoid, tetanus, diphtheria, and certain types of cancers.
• Vaccines have also been responsible for eradication (or near eradication) of deadly diseases such as smallpox and polio.

How are vaccines made?

• There are multiple ways to make vaccines.
• For a killed vaccine, the pathogen can be grown outside the body, purified and inactivated with chemicals. Example: injectable polio vaccine.
• Alternatively, the pathogen can be weakened by repeated culture or by genetic means. Example: oral polio vaccine.
• One of the proteins on the pathogen surface can also be produced artificially and used to raise immunity. An example of this would be the Hepatitis B vaccine.

Stages of vaccine development

Development of vaccines can be simplified into two broad stages:

1. Pre-clinical development is research carried out in lab assays and on animals.  It includes:
   • Identification (discovery) of relevant antigens (e.g. screening)
   • Evaluation of vaccine efficacy in test tubes and animals
2. Clinical development is when the vaccine is first tested in humans.
   • It covers four stages over several years, from initial clinical trials in humans (phase I) right through to introduction and beyond (phase IV).
   • Clinical development is built on rigorous ethical principles of informed consent from volunteers, with an emphasis on vaccine safety as well as efficacy.

Phases of clinical development

• Vaccines take several years to develop, and their development typically proceeds through three phases of clinical trials.
• In Phase 1, small groups of people receive the trial vaccine.
• During Phase 2, the clinical study is expanded and the vaccine is given to people who have characteristics (such as age and physical health) similar to those for whom the new vaccine is intended.
• In Phase 3, the vaccine is given to several thousand people and tested for efficacy and safety.
  • During this phase, participants either receive the vaccine or a placebo.
  • The efficacy of the vaccine is determined by comparing the prevalence of infection in the group that was administered the vaccine with the one which received a placebo.
  • The hypothesis that those in the vaccine group will be infected significantly less is thus tested.

Is a virus needed to develop a vaccine against it?

• With modern molecular tools and technologies, isolating a virus is not required to make a vaccine against it.
• The genetic sequence can be obtained directly from infected patients and this can be converted into proteins using various platform technologies.
• This makes it possible to have vaccines ready for hundreds of pathogens that have not yet infected humans, but have the potential to do so in future.

Why in News?

The World Health Organisation (WHO) site lists 10 vaccine candidates for Covid-19 in clinical evaluation and 126 candidate vaccines in preclinical evaluation.