India on Tuesday (Jun 22) declared a new coronavirus variant to be of concern, and said nearly two dozen cases had been detected in three states.
The variant, identified locally as "Delta plus", was found in 16 cases in the state of Maharashtra, Federal Health Secretary Rajesh Bhushan told a news conference.
The ministry said Delta plus showed increased transmissibility and advised states to increase testing.
On Monday, India vaccinated a record 8.6 million people as it began offering free shots to all adults, but experts doubted it could maintain that pace.
"This is clearly not sustainable," Chandrakant Lahariya, an expert in public policy and health systems, told Reuters.
"With such one-day drives, many states have consumed most of their current vaccine stocks, which will affect the vaccination in days to follow."
With the currently projected vaccine supply for the next few months, the maximum daily achievable rate is 4 to 5 million doses, Lahariya added.
The effort has so far covered about 5.5 per cent of the 950 million people eligible, even though India is the world's largest vaccine producer.
A devastating second wave during April and May overwhelmed health services, killing hundreds of thousands. Images of funeral pyres blazing in car parks raised questions over the chaotic vaccine rollout.
Since May, vaccinations have averaged fewer than 3 million doses a day, far less than the 10 million health officials say are crucial to protect the millions vulnerable to new surges.