Patient Education - COVID-19 Vaccines Explained

COVID-19 Vaccines Explained

  • Dr. Ratika Rastogi, Physician
COVID-19 Vaccines Explained


When most people in a community are vaccinated against a disease, the ability of the pathogen to spread is limited. This is called ‘herd’ or ‘indirect’ or ‘population’ immunity. When many people have immunity, this also indirectly protects people who cannot be vaccinated, such as very young babies and those who have compromised immune systems. 

Vaccines work by reducing the risk of infection by training the immune system to recognize and fight pathogens such as viruses or bacteria


  • Pre-clinical studies- Vaccine is tested in animal studies for efficacy and safety, including challenge studies
  • Phase I clinical trial -Small groups of healthy adult volunteers receive the vaccine to test for safety 
  • Phase II clinical trial- Vaccine is given to people who have characteristics (such as age and physical health) similar to those for whom the new vaccine is intended 
  • Phase III clinical trial- Vaccine is given to thousands of people and tested for efficacy and safety 
  • Phase IV post marketing surveillance -Ongoing studies after the vaccine is approved and licensed, to monitor adverse events and to study long-term effects of the vaccine in the population

Several different types of potential vaccines for COVID-19 are in development -

  • Inactivated or weakened virus vaccines, which use a form of the virus that has been inactivated or weakened so it doesn’t cause disease, but still generates an immune response.
  • Protein-based vaccines, which use harmless fragments of proteins or protein shells that mimic the COVID-19 virus to safely generate an immune response.
  • Viral vector vaccines, which use a virus that has been genetically engineered so that it can’t cause disease, but produces coronavirus proteins to safely generate an immune response.
  • RNA and DNA vaccines, a cutting-edge approach that uses genetically engineered RNA or DNA to generate a protein that itself safely prompts an immune response.


Hyderabad-based Bharat Biotech sought emergency use authorization for Covaxin, the country’s indigenous Covid-19 vaccine candidate. 

The American pharma giant Pfizer was the first to seek emergency use authorization for its RNA candidate. 

Pune’s Serum Institute of India sought emergency use authorization for Covishield, a version of the Oxford-AstraZeneca vaccine candidate that it is manufacturing under license .




No. of Dose 




Oxford Uni-AstraZeneca


Viral vector 

(genetically modified virus) 

X 2

  62-90 % 

Regular fridge temperature 





(part of virus genetic code) 

X 2


-20C up 

To 6 months 






X 2 






(Sputnik V) 

Viral vector 

X 2 



Regular fridge temperature

(in dry form)  




Over the course of next year and beyond, India will carry out the largest vaccination drive in its history. Guidelines on administration of the vaccine are based on the crucial question of the likelihood of an adverse event being reported after each dose. Each vaccination will take at least 30 minutes, only 100 shots will be administered in each session. Both government and private healthcare facilities will be roped in to conduct the first round of immunizations for frontline healthcare workers.

If a safe and effective vaccine becomes available, after convincing clinical trials and judicious regulatory approvals, it is best to get vaccinated. 

Until much of next year, we will need to diligently observe public health advisories on masks, physical distancing, hand washing and avoiding super spreader events. The advent of summer may give us some respite but not full release.

So, if 2020 has been the year of despair and depression, 2021 has to be the year of discipline and determination to subdue the virus with all the tools we are gathering or making now.