India expanded its vaccination programme to include all adults over the age of 18 starting 1 May amid a deadly second wave of infections.
The country's healthcare system is overwhelmed with more than 200,000 cases and about 1,300 deaths being reported daily.
India has so far administered more than 127 million doses of a coronavirus vaccine in what is the world's largest inoculation drive.
Despite a promising start in January, India's vaccine drive has been lagging. And the decision to expand it to the biggest cohort of the country's population - 18 to 45 year-olds - comes even as several states report a shortage of doses.
Since the pandemic began, India has confirmed more than 15 million cases and over 180,000 deaths. It has the second-highest number of Covid-19 infections in the world after the United States.
How is the rollout going?
More than 109 million people have received one dose, and over 17 million people have been fully vaccinated after receiving two doses.
But experts say India is unlikely to meet its target of covering 250 million people by July, especially as cases continue to surge.
India launched its vaccination drive on 16 January, restricting it to healthcare workers and frontline staff - a sanitation worker became the first Indian to receive the vaccine. The drive gradually expanded to other age groups - from 1 May, those above 18 years will be the latest group to become eligible.
There are no plans yet to vaccinate pregnant woman and children.
In early April, India declared that it was "the fastest country in the world" to give more than 100 million jabs. It achieved the feat in 85 days, whereas the US took 89 days and China 102 days, the health ministry said.
But shortages have since affected the vaccine drive in many states with second doses being deferred.
India is currently giving jabs of two vaccines - one developed by AstraZeneca with Oxford University (Covishield) and one by Indian firm Bharat Biotech (Covaxin). Both were approved in January ahead of the vaccine rollout.
In April, a third vaccine - Russia's Sputnik V - was approved for use. Several other candidates are at different stages of trials.
India placed a temporary hold on all exports of AstraZeneca to meet domestic demand but the vaccine's maker, Serum Institute of India (SII) recently said its production capacity was "very stressed" and that it was "still short of being able to supply to every Indian".
The federal government has since cleared the way for an "advance payment" of 4,500 crore rupees ($610m; £435m) to Bharat Biotech and SII to boost their supply.
Experts believe India should ramp up vaccination in areas of high transmission and in five states where elections are being held.
Bhramar Mukherjee, a biostatistician at the University of Michigan, told the BBC that India needed to administer 10 million shots daily "instead of being complacent with three million" doses a day.
"I do feel frustrated that India did not roll out the vaccination drive more aggressively while the curve was in its valley," Dr Mukherjee said.
How does India manage such a huge vaccine drive?
For decades now, the country has been running one of the world's largest immunisation programmes that vaccinate tens of millions, including newborns and pregnant women, against various diseases.
So experts believed India was well-prepared for the challenge. But the uptake has been slow because of vaccine scepticism as well as a lack of awareness among the poor or in rural areas.
Many of the poor have little information on how to register themselves and access the vaccine free of cost. Eligible people can book their jabs online or walk in and register at vaccination centres.
Who is paying for the vaccines?
Vaccination is voluntary. State-run clinics and hospitals are offering free jabs, but people can also pay 250 rupees ($3.4; £2.4) a dose at private facilities.
From 11 April, the government allowed people to get paid jabs at private and state-run workplaces.
The government is spending around $5bn to provide free doses at state-run clinics, public health centres and hospitals.
Have there been 'adverse events' after vaccination?
People can experience side effects from vaccines.
India has a 34-year-old surveillance programme for monitoring "adverse events" following immunisation. Experts say a failure to transparently report such incidents could lead to fear-mongering around vaccines.
India has so far reported 18,904, "adverse events" after vaccination. Most of these events were "minor" - anxiety, vertigo, giddiness, dizziness, fever, and pain - and all patients had recovered, the government said.
The surveillance programme has examined 617 cases of "severe adverse events", including 180 deaths after vaccination until March, according to reports.
It found the "deaths happened in cases where the person had underlying conditions, including heart problems, high blood pressure and diabetes".
Charts by Shadab Nazmi