A third coronavirus vaccine has been approved for use in India amid a deadly second wave of infections.
Russia's Sputnik V has been deemed to be safe, and works in a way similar to the Oxford-AstraZeneca jab which is being made in India as Covishield.
Sputnik V gives around 92% protection against Covid-19, late stage trial results published in The Lancet reveal.
India has so far given more than 100 million doses of two approved vaccines - Covishield and Covaxin.
Sputnik V's approval came as India overtook Brazil to become the country with the second-highest number of cases globally.
With the total case tally of more than 13.5 million cases, India is now only behind the United States which has reported more than 31 million cases. With 13.4 million cases, Brazil is now at number three.
India aims to vaccinate 250 million "priority people" by the end of July. But experts say that the pace of vaccination has been slow and unless the drive is scaled up, the target could be missed.
On Tuesday the government decided to give emergency approvals to vaccines already in use in other countries. It said the decision was taken to "expand the basket of vaccines for domestic use and hasten the pace and coverage of vaccination".
It added that vaccines that have been approved by regulators in the US, the UK, European Union and Japan would be given fast-track approvals in India. The health ministry added that the first 100 recipients of such vaccines would be monitored for seven days before wider rollout is allowed.
This means that vaccines made by Pfizer and Moderna could become available for Indians. However, the government is yet to give any details.
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.
When will Sputnik V be available in India?
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 while the third phase of the trial was still underway, sparking scepticism and questions from experts.
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 was the controversy around Covaxin?
It all began when the regulator in January said the vaccine had been approved for "restricted use in emergency situations in public interest as an abundant precaution, in clinical trial mode, especially in the context of infection by mutant strains".
Experts wondered how a vaccine was cleared for emergency use by millions of vulnerable people when its trials were still underway. The All India Drug Action Network at the time said that it was "baffled to understand the scientific logic" to approve "an incompletely studied vaccine". It said that there were "intense concerns arising from the absence of the efficacy data".
Both the manufacturer and drug regulator had defended Covaxin, saying it was "safe and provides a robust immune response".
Bharat Biotech had said that Indian clinical trial laws allowed "accelerated" authorisation for use of drugs after the second phase of trials for "unmet medical needs of serious and life-threatening diseases in the country". It had promised to provide efficacy data for the vaccine by February, which it has now done.
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 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.
But 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.
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
Which countries are signing up for India's vaccines?
India has shipped 64 million doses of vaccines to 86 countries in Latin America, the Caribbean, Asia and Africa. The recipient countries include UK, Canada, Brazil and Mexico.
Both Covishield and Covaxin have been exported - some in the form of "gifts", others in line with commercial agreements signed between the vaccine makers and the recipient nations, and the rest under the Covax scheme, which is led by the World Health Organization (WHO) and hopes to deliver more than two billion doses to people in 190 countries in less than a year.
But in March, India placed a temporary hold on all exports of the Oxford-AstraZeneca vaccine. The government said rising cases meant domestic demand was expected to pick up and so the doses were needed for India's own rollout.