India became the third country to surpass 400,000 Covid-19 deaths, official data showed on Friday, as the country's vast vaccination drive slows.

The total number of Covid-19 deaths reported in the country is 400,312, according to the health ministry — behind only the US and Brazil — and nearly 30.5 million cases have been reported in total.

Many experts suspect India's actual death toll is more than a million, after a devastating spike in cases in April and May that overwhelmed hospitals.

The surge was blamed on the Delta variant and the government's complacency after Prime Minister Narendra Modi declared victory over the virus in January.

Daily case numbers have since decreased significantly and many restrictions on activity have been lifted, raising fears of a new spike in coming months.

The government aims to fully vaccinate all of the country's 1.1 billion adults this year, but because of shortages, administrative confusion and hesitancy, only around five per cent have been administered two doses so far.

On June 21, the government tried to jumpstart the drive by making vaccines free for all adults, leading to a surge in demand with more than nine million shots being given in a day.

Daily inoculation rates have since slowed again, however, averaging just over four million per day over the past week, according to government figures.

A government affidavit filed with the Supreme Court this week slashed the number of doses the government expects to be available between August and December to 1.35 billion, from a previous projection of 2.16 billion, according to media reports.

The filing mentioned five kinds of vaccine, down from eight in its forecast in May, the reports said, and cut the number of predicted AstraZeneca doses to 500 million from 750 million previously.

On Tuesday, India approved the Moderna vaccine for domestic use.

Indian drugmaker Zydus Cadila on Thursday said it had applied for approval for its plasmid DNA-based vaccine after trials showed an efficacy rate of 67 percent.

Cadila chief Sharvil Patel told a news conference the firm hoped to produce 50 million doses by the end of the year with monthly output of 10 million.