India's vaccinations over the next few weeks could fall short of the blistering pace set on the first day of a federal campaign, unless it makes inroads in a vast hinterland and bridges a shortage of doses, experts said.

The 8.6 million doses injected on Monday represented a record two-fold 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.

“With the currently projected vaccine supply for the next few months, the maximum daily achievable rate is 4 million to 5 million a day.”