An ambitious effort by the World Health Organisation (WHO) to calculate the global death toll from the coronavirus pandemic has found that vastly more people died than previously believed — a total of about 15 million by the end of 2021, more than double the official total of six million reported by countries individually.

But the release of the staggering estimate — the result of more than a year of research and analysis by experts around the world and the most comprehensive look at the lethality of the pandemic to date — has been delayed for months because of objections from India, which disputes the calculation of how many of its citizens died and has tried to keep it from becoming public.

More than a third of the additional nine million deaths are estimated to have occurred in India, where the government of Prime Minister Narendra Modi has stood by its own count of about 5.2 lakh deaths. The WHO will show the country’s toll is at least 40 lakh, according to people familiar with the numbers who were not authorised to disclose them, which would give India the highest tally in the world, they said. The New York Times was unable to learn the estimates for other countries.

The WHO calculation combined national data on reported deaths with new information from localities and household surveys, and with statistical models that aim to account for deaths that were missed. Most of the difference in the new global estimate represents previously uncounted deaths, the bulk of which were directly from Covid; the new number also includes indirect deaths, like those of people unable to access care for other ailments because of the pandemic.

The delay in releasing the figures is significant because the global data is essential for understanding how the pandemic has played out and what steps could mitigate a similar crisis in the future. It has created turmoil in the normally staid world of health statistics -- a feud cloaked in anodyne language is playing out at the United Nations Statistical Commission, the world body that gathers health data, spurred by India's refusal to cooperate.

“It’s important for global accounting and the moral obligation to those who have died, but also important very practically. If there are subsequent waves, then really understanding the death total is key to knowing if vaccination campaigns are working," said Prabhat Jha, director of the Centre for Global Health Research in Toronto and a member of the expert working group supporting the WHO's excess death calculation. "And it's important for accountability."

To try to take the true measure of the pandemic's impact, the WHO assembled a collection of specialists including demographers, public health experts, statisticians and data scientists. The Technical Advisory Group, as it is known, has been collaborating across countries to try to piece together the most complete accounting of the pandemic dead.

The New York Times spoke with more than 10 people familiar with the data. The WHO had planned to make the numbers public in January but the release has continually been pushed back.

Recently, a few members of the group warned the WHO that if the organisation did not release the figures, the experts would do so themselves, three people familiar with the matter said.

A WHO spokesperson, Amna Smailbegovic, told The New York Times: "We aim to publish in April."

Samira Asma, the WHO's assistant director-general for data, analytics and delivery for impact, who is helping with the calculation, said the release of the data had been "slightly delayed" but said it was "because we wanted to make sure everyone is consulted."

India insists that the WHO's methodology is flawed. "India feels that the process was neither collaborative nor adequately representative," the Indian government said in a statement to the United Nations Statistical Commission in February.

It also argued that the process did not "hold scientific rigour and rational scrutiny as expected from an organisation of the stature of the World Health Organisation".

India's health ministry did not respond to requests for comment.

India is not alone in undercounting pandemic deaths: the new WHO numbers also reflect undercounting in other populous countries such as Brazil and Indonesia.

Asma noted that many countries had struggled to accurately calculate the pandemic's impact. Even in the most advanced countries, she said, "I think when you look under the hood, it is challenging."

At the start of the pandemic there were significant disparities in how quickly the various US states were reporting deaths, she said, and some were still collecting the data via fax.

India brought a large team to review the WHO data analysis, she said, and the agency was glad to have them do it, because it wanted the model to be as transparent as possible.

India's work on vaccination has won praise from experts globally, but its public health response to Covid has been criticised for overconfidence.

Modi boasted in January 2021 that India had "saved humanity from a big disaster". A couple of months later, his health minister declared that the country was "in the endgame of Covid-19". Complacency set in, leading to missteps and attempts by officials to silence critical voices within elite institutions.

Then, in April 2021, a devastating second wave hit. Hospitals had to turn patients away and oxygen ran out. But many deaths went uncounted.

Science in India has been increasingly politicised over the course of the pandemic. In February, India's junior health minister criticised a study published in the journal Science that estimated the country's Covid death toll to be six to seven times the official number. In March, the government questioned the methodology of a study published in The Lancet that estimated India's deaths at four million.

"Personally, I have always felt that science has to be responded with science," said Bhramar Mukherjee, a professor of biostatistics at the University of Michigan School of Public Health who has been working with the WHO to review the data.

"If you have an alternative estimate, which is through rigorous science, you should just produce it. You cannot just say, 'I am not going to accept it'."

India has not submitted its total mortality data to the WHO for the past two years, but the organisation's researchers have used numbers gathered from at least 12 states, including Andhra Pradesh, Chhattisgarh and Karnataka, which experts say show at least four to five times more deaths as a result of Covid-19.

Jon Wakefield, a professor of statistics and biostatistics at the University of Washington who played a key role in building the model used for the estimates, said an initial presentation of the WHO global data was ready in December.

"But then India was unhappy with the estimates. So then we've subsequently done all sorts of sensitivity analyses --- the paper's actually a lot better because of this wait because we've gone overboard in terms of model checks and doing as much as we possibly can given the data that's available," Wakefield said. "And we're ready to go."

The numbers represent what statisticians and researchers call "excess mortality" - the difference between all deaths that occurred and those that would have been expected to occur under normal circumstances. The WHO's calculations include deaths caused directly by Covid, deaths of people from conditions complicated by Covid, and deaths of those who did not have Covid but needed treatment they could not get because of the pandemic. The calculations also take into account the expected deaths that did not occur because of Covid restrictions, such as those from traffic accidents.

Calculating excess deaths globally is a complex task. Some countries have closely tracked mortality data and supplied it promptly to the WHO. Others have supplied only partial data, and the agency has had to use modelling to round out the picture.

To produce mortality estimates for countries with partial or no death data, the experts in the advisory group used statistical models and made predictions based on country-specific information such as containment measures, historical rates of disease, temperature and demographics to assemble national figures and, from there, regional and global estimates.

Besides India, there are other large countries, such as Russia and China, where the data is uncertain.

India's effort to stall the report's release makes it clear that pandemic data is a sensitive issue for the Modi government. "It is an unusual step," said Anand Krishnan, a professor of community medicine at the All India Institute of Medical Sciences in New Delhi who has also been working with the WHO to review the data. "I don't remember a time when it has done so in the past."

New York Times News Service