It's a question many of us have long had on our minds: what's the real number of deaths globally caused by the coronavirus pandemic? 

The World Health Organisation (WHO) is in the middle of an ambitious effort to answer this – but it's facing a big problem. 

Experts who have been researching the answer for the WHO over the past year believe that the true Covid-19 death toll up to the end of 2021 sits at a staggering 15 million – although this data has not yet been publicly released, the New York Times reported. 

That's more than double the official toll that countries have reported separately, at six million. 

There are claims that attempts at obtaining accurate calculations have been hindered by some countries that have under-reported deaths such as India, which has allegedly sparked months-long delays at publicly releasing this data due to it disputing the death toll. 

According to the Times, who spoke to 10 people familiar with the WHO data: 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 520,000. 

It reported the WHO would show the country's toll is at least four million, according to people familiar with the numbers who were not authorised to disclose them, which would give India the highest tally in the world. 

The WHO used national data on deaths combined with new information from areas and surveys from households to come up with the numbers. They also used statistical models to account for missed fatalities. 

Also included in the new worldwide estimates are indirect deaths, including of those who were unable to access care for other health concerns due to the pandemic. 

"It's important for global accounting and the moral obligation to those who have died, but also important very practically," Dr Prabhat Jha, director of the Centre for Global Health Research in Toronto. He is also a member of the working group supporting the WHO's excess death calculation. 

"If there are subsequent waves, then really understanding the death total is key to knowing if vaccination campaigns are working. 

"And it's important for accountability." 

Professor Sanjeev Bagai from New Delhi slammed the WHO for their methodology in calculating the death toll and for singling out India, reported India's Mirror Now. 

"There is no collaborative effort in a scientific basis across various countries," Bagai said of the research "limitations". 

"All countries cannot be treated alike. Almost 80 per cent of the world's countries don't have the health records which are digitalised, it's almost virtually impossible to get a near accurate figure. 

"Under-reporting or underestimating of Covid deaths is a global phenomena, it's not just India centric … The cruellest joke on the world is that China has reported less than 5000 deaths, I mean this is ridiculous." 

He called for a more rational approach, saying there are limitations worldwide due to such issues as home tests not being reported, and deaths not being counted as primary or secondary to Covid. 

Volunteers unload food supplies from a truck at a gated community after Shanghai imposed a citywide lockdown to halt the spread of Covid. 

In China, the overall death rate in Wuhan was believed to be 50 per cent higher than expected in the first quarter of 2020, according to a group of government researchers who found that heart disease and diabetes deaths spiked during the lockdown and the increased deaths were due to obstacles in seeking help. 

Indonesia and Egypt are also among the other highly populated countries who were noted as having under-counted pandemic fatalities. In addition, Russia had reported 300,000 Covid deaths by the end of 2021, but it's believed to have an excess mortality rate of more than one million people.