About 3.2 million people could have died from Covid in India by September last year, six-seven times higher than reported officially, according to a study based on one independent and two government data sources.

The research, published in the journal Science on Thursday, used an independent nationally representative survey of 137,289 adults in all states and union territories interviewed from March 2020 to July 2021.

An international team of researchers led by Prabhat Jha, a professor at the University of Toronto in Canada, found that COVID-19 constituted 29 per cent of deaths from June 2020 to July 2021, corresponding to 3.2 million (32 lakh) deaths, of which 2.7 million (27 lakh) occurred in April-July 2021.

A sub-survey of 57,000 adults showed similar increases in mortality with COVID-19 and non-COVID-19 deaths peaking similarly, they said.

Two government data sources found that, when compared to pre-pandemic periods, all-cause mortality or death from any cause -- was 27 per cent higher in 0.2 million health facilities and 26 per cent higher in civil registration deaths in 10 states, the researchers said.

Both increases occurred mostly in 2021, according to the study.

The analyses find that India's cumulative COVID deaths by September 2021 were 6-7 times higher than reported officially, the authors of the study said.

The researchers noted that India's reported COVID-19 death totals are widely believed to be under-reported because of factors like incomplete certification of COVID deaths and misattribution to chronic diseases.

The first study is mortality reported in a nationally representative telephone survey conducted by CVoter, an independent, private polling agency.

In addition, the researchers studied the Government of India's administrative data on national facility-based deaths and civil registration system (CRS) deaths in 10 states.

As of January 1, 2022, India has reported over 35 million cases of SARS-CoV-2, second only to the US, while its official cumulative Covid death count stands at 0.48 million (4.8 lakh).

If our findings are confirmed, this may require substantial upward revision of the World Health Organization's estimates of cumulative global COVID mortality, which as of January 1, 2022, stood at 5.4 million, the authors said.

The researchers said model-based estimates of cumulative COVID deaths through June 2021 in India range from a few hundred thousand to over four million, with most suggesting a substantial official undercount.

In order to better estimate India's COVID deaths, alternative approaches to model-based estimates are needed, they said.

The World Health Organization has recognised counts of recorded increases in all-cause mortality during peak pandemic transmission -- which are likely nearly all caused by COVID infection -- as a crude but useful method to track the pandemic.

To fill the gaps in India's national-level COVID death estimates, the researchers quantified COVID mortality in India using one independent and two government data sources.

The results show consistency between the size of viral waves and Covid deaths.

Our study finds that Indian COVID deaths are substantially greater than estimated from official reports,' the authors added.

The team includes researchers from the Center For Voting Opinions and Trends in Election Research, Noida, Uttar Pradesh, Indian Institute of Management Ahmedabad, Ahmedabad, Gujarat, Development Data Lab, Washington, US, and Department of Economics, Dartmouth College.