When the vaccine is injected into a patient, it prompts the immune system to start making antibodies and primes it to attack any coronavirus infection.
The jab is administered in two doses given between four and 12 weeks apart. It can be safely stored at temperatures of 2C to 8C and can easily be delivered in existing health care settings such as doctors' surgeries.
The jab developed by Pfizer-BioNTech, which is currently being administered in several countries, must be stored at -70C and can only be moved a limited number of times - a particular challenge in India, where summer temperatures can reach 50C.
How effective is Covishield?
International clinical trials of the Oxford-AstraZeneca vaccine showed that when people were given a half dose and then a full dose, effectiveness hit 90%.
But there was not enough clear data to approve the half-dose, full-dose idea.
However, unpublished data suggests that leaving a longer gap between the first and second doses increases the overall effectiveness of the jab - in a sub-group given the vaccine this way it was found to be 70% effective after the first dose.
The Serum Institute (SII), the Indian maker of the vaccine, says Covishield is "highly effective" and backed by phase III trial data from Brazil and United Kingdom. Clinical trials are a three-phased process to determine whether the vaccine induces good immune responses and whether it causes any unacceptable side-effects.
pgslotBut patients' rights group, All India Drug Action Network, says its approval has been rushed because the manufacturer has not completed a "bridging study" of the vaccine on Indians.