The 8.6 million doses injected on Monday represented a record twofold jump as India kicked off free inoculation for all adults, reversing a policy for individual states and hospitals to buy vaccines for those aged 18 to 44.

"This is clearly not sustainable," Chandrakant Lahariya, an expert in public policy and health systems, told Reuters on Tuesday.

"With the currently projected vaccine supply for the next few months, the maximum daily achievable rate is four million to five million a day."

The inoculation effort had covered just about 5.5 per cent of all the 950 million people eligible, even though India is the world's largest producer of vaccines.

A devastating second wave of infections during April and May overwhelmed medical staff and facilities, killing hundreds of thousands.

Since May, India has distributed an average of fewer than three million doses each day, far less than the 10 million officials say is crucial to protect the millions vulnerable to new surges.

Particularly in the countryside, where two-thirds of a population of 1.4 billion lives and the healthcare system is often overstretched, the drive has faltered, experts say.

Delhi-based epidemiologist Rajib Dasgupta said maintaining the pace of the vaccination effort would prove particularly challenging when it came to injecting the younger population in such "under-served" areas.

India reported 42,640 new infections in the past 24 hours, its lowest figure since March 23, along with 1167 deaths.

Its tally of infections now stands at 29.98 million, with a death toll of 389,302, health ministry data showed.

Australian Associated Press