India on Saturday will launch one of the world's biggest vaccination programmes with vaccines manufactured in the country - one developed by Oxford University and AstraZeneca, the other by Bharat Biotech International with a state-run institute. First to get the vaccine will be 3 crore health and other frontline workers, followed by about 27 crore older than 50 or considered high-risk. Here is a look at the technology behind the two drugs that will be given out.
Developed by the University of Oxford and British-Swedish pharmaceutical company AstraZeneca and manufactured by the Pune-based Serum Institute of India, Covishield has emerged as the most promising vaccine for countries like India where cost and logistics are a significant consideration.
Unlike the Pfizer and Moderna vaccines, which use a newer approach called mRNA making them more complex, fragile and require ultracold temperatures, Covishield is a vector vaccine which are slower but cheaper and importantly, can be kept stable for six months at standard refrigerator temperatures.
The vaccine uses a weakened version of a common cold virus called an adenovirus that affects chimpanzees but doesn't infect humans. It attaches to cells and injects DNA that tells them to make the coronavirus spike protein - the structures on the surface of the coronavirus, giving it that studded appearance.
This catches the attention of the immune system which recognizes it as foreign and builds up a defence to attack the real coronavirus when an actual infection occurs.
Early results published by Oxford-AstraZeneca researchers in the UK showed that the vaccine was around 70 per cent effective.
Developed by Indian biotechnology company Bharat Biotech and country's top clinical research body Indian Council of Medical Research (ICMR), Covaxin is the second vaccine to have been approved by the government.
It is an inactivated vaccine - one of the oldest methods for vaccinating people - meaning it uses whole, inactivated viruses that are injected to trigger an immune response. These whole batches of coronavirus must be grown, "killed" using a chemical or heat and then made into a vaccine, making it a longer process.
Chinese companies Sinovac and Sinopharm are also using the same technique for their coronavirus vaccines.
There have been some concerns about Covaxin because its efficacy has not yet been proven in Phase 3 clinical trials, unlike Covishield and vaccines by Pfizer and Moderna. While the first two phases of vaccine trials generally find out if they are safe, the third phase usually finds out if the vaccine is effective or not.
Phase 3 trials for Covaxin are still underway and this has worried some health experts who consider it rushed, as the vaccine has only limited, "clinical-trial mode" approval.
Experts at India's drug regulator this month recommended stricter monitoring for the vaccine, as is done during clinical trials and the government has ordered 55 lakh or only half as many doses of Covaxin as Covishield. Brazil this week became the first foreign country to sign a deal to buy the Indian shot.
Among the vaccines that are yet to come to India are the ones made by American pharmaceutical firm Pfizer and its German-based partner BioNTech and another by American company Moderna which are being used in US, UK, and other countries.
Both use messenger ribonucleic acid or mRNA, a sort of genetic software that asks cells to make a piece of the coronavirus spike protein, putting them on guard against infections. The approach can produce a vaccine quickly because scientists need just the genetic sequence of the virus, making it a frontrunner for pandemic vaccine years ago.
Russia's Sputnik V vaccine, which has been cleared by eight countries so far, is undergoing last-stage trials in India after completing a Phase 2 study. An adenoviral vector vaccine like Covishield, it is expected to soon apply for emergency-use approval in the country.
American multinational Johnson & Johnson's vaccine arm Janssen Pharmaceuticals has also said it is on track to roll out its single-shot coronavirus vaccine in March and expects to have clear data on how effective it is by the end of this month or early February. It too is a vector vaccine.
(With inputs from agencies).