India’s death toll from COVID-19 may be up to ten times higher than the official count of nearly 415,000, making the country the world’s worst-affected from the coronavirus since the onset of the pandemic, according to an American research group.

In a study published on Tuesday, which analysed data from the start of the pandemic to June this year, the Centre for Global Development estimates between 3.4 to 4.7 million people have died from the infection in the country.

It is the highest estimated death toll so far for the nation of 1.3 billion people, which was crippled by a ferocious wave of cases, fuelled by the more contagious Delta Variant, in April and May.

“True deaths are likely to be in the several millions, not hundreds of thousands, making this arguably India's worst human tragedy since partition and independence,” the research said.

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Violent bloodshed followed the sub-continent’s partition in 1947 into Hindu-dominated India and Muslim-dominated Pakistan, with some estimates projecting up to two million people died in the sectarian crisis.

The country’s official death toll due to the virus is the third highest in the world, behind only Brazil (over 542,000) and the United States (over 609,000). India has logged the highest single-day rise in cases and deaths in the world - over 400,000 and 4,500 respectively.

But experts have doubted both the number of infections and the death toll that the health authorities have reported. A number of Indian states have revised their respective tolls recently, adding thousands of “backlog” deaths.

The Center for Global Development report was based on estimating "excess mortality", the number of extra people who died compared with pre-crisis figures.

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The authors, including former chief economic advisor Arvind Subramanian, did this partly by analysing death registrations in some states as well as a recurring national economic study.

They also compared surveys of the spread of COVID-19 in India with international death rates.

The researchers, which also included a Harvard University expert, acknowledged that estimating mortality with statistical confidence was difficult.

"(But) all estimates suggest that the death toll from the pandemic is likely to be an order of magnitude greater than the official count," they said.

Christophe Guilmoto, a specialist in Indian demography at France's Research Institute for Development, this month estimated that the death toll was nearer 2.2 million by late May.

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India's death rate per million was nearly half the world average and Guilmoto said "such a low figure contradicts the apparent severity of a crisis that has struck most Indian families across the country".

Guilmoto's team concluded that only one coronavirus death in seven was recorded.

A model by the US-based Institute for Health Metrics and Evaluation estimated that the COVID toll could be more than 1.25 million.

India's health ministry last month slammed The Economist magazine for publishing a story that said excess deaths were between five and seven times higher than the official toll, calling it "speculative" and "misinformed".

A World Health Organization (WHO) report in May said up to three times more people had died around the globe during the pandemic -- from coronavirus or other causes -- than indicated by official statistics.