The second phase of the COVID-19 vaccination drive has already begun in India, and many people are still unaware of how the two current vaccines – Covaxin and Covishield – and latest Sputnik V are different from each other.
What do we know about Sputnik V?
The vaccine, developed by Moscow's Gamaleya Institute, initially generated some controversy after being rolled out before the final trial data had been released.
But scientists say its benefits have now been demonstrated.
It uses a cold-type virus, engineered to be harmless, as a carrier to deliver a small fragment of the coronavirus to the body.
Safely exposing the body to a part of the virus's genetic code in this way allows it to recognise the threat and learn to fight it off, without the risk of becoming ill.
After being vaccinated, the body starts to produce antibodies especially tailored to the coronavirus.
This means that the immune system is primed to fight coronavirus when it encounters it for real.
It can be stored at temperatures of between 2 and 8C degrees (a standard fridge is roughly 3-5C degrees) making it easier to transport and store.
But it has a different second dose
Unlike other similar vaccines, the Sputnik jab uses two slightly different versions of the vaccine for the first and the second dose - given 21 days apart.
They both target the coronavirus's distinctive "spike", but use different vectors - the neutralised virus that carries the spike to the body.
The idea is that using two different formulas boosts the immune system even more than using the same version twice - and may give longer-lasting protection.
As well as proving effective, it was also safe with no serious reactions linked to the vaccine during the trial.
Some side-effects to a vaccine are expected, but these are usually mild, including a sore arm, tiredness and a bit of a temperature. There were no deaths or serious illnesses in the vaccinated group linked to the jab.
Sputnik V has been approved so far in 60 countries, including Argentina, Palestinian territories, Venezuela, Hungary, UAE and Iran.
The Russian Direct Investment Fund (RDIF), which is marketing the vaccine, has signed deals to produce more than 750 million doses of Sputnik V in India with six domestic vaccine makers, according to reports.
Hyderabad-based pharmaceutical major Dr Reddy's Laboratories will be importing the first batch of 125 million doses to India during this quarter.
Supplies will be ramped up only next quarter when six Indian firms begin making the vaccine under the supervision of Dr Reddy's.
Until then, India will mostly depend on two previously approved candidates, Covaxin and Covishield.
So what do we know about Covaxin?
Covaxin is an inactivated vaccine which means that it is made up of killed coronaviruses, making it safe to be injected into the body.
Bharat Biotech, a 24-year-old vaccine maker with a portfolio of 16 vaccines and exports to 123 countries, used a sample of the coronavirus, isolated by India's National Institute of Virology.
When administered, immune cells can still recognise the dead virus, prompting the immune system to make antibodies against the pandemic virus.
The two doses are given four weeks apart. The vaccine can be stored at 2C to 8C.
The vaccine has an efficacy rate of 81%, preliminary data from its phase 3 trial shows.
India's regulators gave the vaccine an emergency approval in January.
Bharat Biotech says it has a stockpile of 20 million doses of Covaxin, and is aiming to make 700 million doses out of its four facilities in two cities by the end of the year.
What about Covishield?
The Oxford-AstraZeneca vaccine is being manufactured locally by the Serum Institute of India, the world's largest vaccine manufacturer. It says it is producing more than 60 million doses a month.
The vaccine is made from a weakened version of a common cold virus (known as an adenovirus) from chimpanzees. It has been modified to look more like coronavirus - although it can't cause illness.
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 12 and 16 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.
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%.
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.
Any other vaccine candidates?
The other candidates which are in different stages of trials in India to test safety and efficacy include:
ZyCov-Di, being developed by Ahmedabad-based Zydus-Cadila
A vaccine being developed by Hyderabad-based Biological E, the first Indian private vaccine-making company, in collaboration with US-based Dynavax and Baylor College of Medicine
Hyderabad-based Biological E to produce the vaccine developed by US firm Johnson & Johnson
HGCO19, India's first mRNA vaccine made by Pune-based Genova in collaboration with Seattle-based HDT Biotech Corporation, using bits of genetic code to cause an immune response
A nasal vaccine by Bharat BioTech
A second vaccine being developed by Serum Institute of India and American vaccine development company Novavax