WASHINGTON (BLOOMBERG) - The pandemic's death toll may be three times higher than official Covid-19 records suggest, according to a study that found stark differences across countries and regions.

As many as 18.2 million people probably died from Covid-19 in the first two years of the pandemic, researchers found in the first peer-reviewed global estimate of excess deaths.

They pointed to a lack of testing and unreliable mortality data to explain the discrepancy with official estimates of roughly 5.9 million deaths.

"At the global level, this is quite the biggest mortality shock since the Spanish flu," said Dr Christopher J.L. Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington, where the study was conducted. Covid-19 drove a 17 per cent jump in deaths worldwide, he said in an interview.

The findings, published in the Lancet medical journal, focused on excess deaths to avoid under-counting and assess the extent of the pandemic's devastation.

While deaths continued to accumulate, the scientists compared the mortality between Jan 1, 2020 and Dec 31, 2021 to comparable data for the prior years.

The evidence suggests the mortality surge is a direct result of Covid-19, the researchers said.

But some deaths may also have occurred indirectly, they said, caused by a lack of access to health care and other essential services during the pandemic, or from behavioural shifts that led to suicide or drug abuse.

"Studies from several countries, including Sweden and the Netherlands, suggest Covid-19 was the direct cause of most excess deaths," said Dr Haidong Wang, an associate professor of health metric sciences at the Seattle-based institute, in a statement.

"Understanding the true death toll from the pandemic is vital for effective public health decision-making."

Improving data on deaths can give governments a clearer picture of how best to direct efforts to protect their citizens, said Dr Jennifer Ellis, who leads the Data for Health programme at Bloomberg Philanthropies that works with low- and middle-income countries to strengthen information gathering.

Bloomberg Philanthropies is the charitable organization set up by Mr Michael Bloomberg, founder and majority owner of Bloomberg News' parent Bloomberg LP.

"The pandemic has made clear that keeping track of how many people are dying, and the reason for those deaths, is vital for governments to formulate better-informed policies and improved health outcomes," Dr Ellis said.

Only 36 countries have released cause-of-death data for 2020 so far.

The researchers used weekly or monthly data on deaths from all causes in the past two years and up to 11 prior years for 74 countries and 266 states and provinces through searches of government websites, mortality databases and the European Statistical Office.

A statistical model was used to predict excess deaths for countries that didn't report weekly or monthly data. Excess deaths were 9.5 times higher than reported in South Asia and 14.2 times higher in sub-Saharan Africa, the researchers found.

Because of its large population, India alone accounted for an estimated 22 per cent - or 4.1 million - of the global deaths. The US and Russia were the next highest with 1.1 million each, followed by Mexico, Brazil and Indonesia.

In the end, the scientists estimated there was an excess of 120 deaths for every 100,000 people around the world.

The study found 21 countries had estimated mortality rates higher than 300 per 100,000, led by Bolivia and Bulgaria.

Mask-wearing, physical distancing and other public health measures led to a decline in other communicable diseases, which reduced mortality in some countries.

Places with the lowest estimated excess mortality rate were Iceland, Australia, Singapore and New Zealand.

The study found that the prevalence of obesity and older age in a population were two of the biggest determinants of excess mortality, Dr Murray said.

"Countries that have high obesity have really had much worse excess mortality," he said. And "age is just such an overwhelming risk factor for Covid that it's not surprising that older societies in North America, Europe and Eastern Europe have had much higher excess death rates."