Recipients include 30 million doctors, nurses and other frontline workers to be followed by 270 million others, who are either aged over 50 or have illnesses that make them vulnerable to Covid-19.
India has started inoculating health workers in what is likely the world's largest Covid-19 vaccination campaign, joining the ranks of wealthier nations where the effort is already well under way.
The first dose of a vaccine was administered on Saturday to Manish Kumar, a health worker, at All Indian Institute of Medical Sciences in the capital New Delhi after Prime Minister Narendra Modi kicked off the campaign with a nationally televised speech.
Priority groups across the vast country, from the Himalayan mountains to the Andaman Islands in the Bay of Bengal, began receiving it shortly after.
"We are launching the world’s biggest vaccination drive and it shows the world our capability," Modi said in his address.
He implored citizens to keep their guard up and not to believe any "rumors about the safety of the vaccines."
The country is home to the world's largest vaccine makers and has one of the biggest immunisation programmes. But there is no playbook for the enormity of the challenge.
Indian authorities hope to give shots to 300 million people, roughly the population of the US and several times more than its existing programme that targets 26 million infants.
The recipients include 30 million doctors, nurses and other front-line workers to be followed by 270 million others, who are either aged over 50 or have illnesses that make them vulnerable to Covid-19.
Politicians not prioritised
It was not clear if Modi, 70, has taken the vaccine himself like other world leaders as an example of the shot’s safety. His government has said politicians will not be considered priority groups in the first phase of the rollout.
Health officials haven't specified what percentage of the nearly 1.4 billion people will be targeted by the campaign.
But experts say it will almost certainly be the largest such drive globally.
The sheer scale has its obstacles.
For instance, India plans to rely heavily on a digital platform to track the shipment and delivery of vaccines.
But public health experts point out that the internet remains patchy in large parts of the country, and some remote villages are entirely unconnected.
Around 100 people will be vaccinated in each of the 3,006 centres across the country on the first day, the Health Ministry said this week.
Concerns over local Bharat Biotech vaccine
India gave a nod for the emergency use of two vaccines, one developed by Oxford University and UK-based drugmaker AstraZeneca, and another by Indian company Bharat Biotech, on January 4.
Cargo planes flew 16.5 million shots to different Indian cities last week.
Health experts worry that the regulatory shortcut taken to approve the Bharat Biotech vaccine without waiting for concrete data that would show its efficacy in preventing illness from the coronavirus could amplify vaccine hesitancy.
At least one state health minister has opposed its use.
India's Health Ministry has bristled at the criticism and says the vaccines are safe, but maintains that health workers will have no choice in deciding which vaccine they would get themselves.
According to Dr S P Kalantri, the director of a rural hospital in Maharashtra, India's worst-hit state, such an approach was worrying because he said the regulatory approval was hasty and not backed by science.
"In a hurry to be populist, the government (is) taking decisions that might not be in the best interest of the common man," Kalantri said.
'Let the politicians get the vaccine first'
Anecdotal evidence suggests that approval of Indian giant Bharat Biotech's vaccine Covaxin without data from Phase 3 human trials – as well as the death of a trial participant – has further eroded trust in the inoculation drive.
With Covaxin still in "trial mode", Indians being given the vaccine on Saturday were being asked to sign consent forms.
The other jab to be given approval is Covishield, a version of AstraZeneca and Oxford University's shot made by India's Serum Institute, the world's largest vaccine manufacturer.
"I think it is all very fishy," said housewife Prerna Srivastava, 41. "Let the politicians get the vaccine first."
"The problem is that no one trusts this government," echoed father-of-five Liaquat Ali, 51, in Mumbai.
"I have no idea when I or my family will get the vaccine, it will take at least a year since this is a huge country. Eventually, everyone will get it but the rich will get it first."
Myths and rumours
More than 150,000 Indians have died from Covid-19 and the economy is one of the worst-hit worldwide, with millions losing their livelihoods.
New infection rates have fallen sharply in recent months but experts are concerned a new wave might hit, fuelled by a string of recent mass religious festivals.
And as in other countries, there is scepticism about the vaccine, fuelled by a torrent of hoaxes and baseless rumours online about the virus.
For example, multiple Facebook and Twitter posts shared hundreds of times –– debunked by AFP Fact Check –– claimed no vegetarian had died from Covid.
Others accused India's Muslim minority of deliberately spreading the virus with hashtags like "#CoronaJihad", or denounced the pandemic as a cover for a plan to implant trackable microchips.