The 2009 H1N1 "swine flu" epidemic killed up to 203,000 people across the globe -- a death toll 10 times greater than initially estimated by the World Health Organization, researchers say.
In a study published Tuesday in the journal Plos Medicine, epidemiologists used data on respiratory deaths in 20 nations to calculate a global mortality rate for the pandemic.
Prior to this research, the WHO counted just 18,631 lab-confirmed cases of H1N1, a viral infection of the airways.
"This study confirms that the H1N1 virus killed many more people globally than originally believed," read a statement from Lone Simonsen, a research professor in the Department of Global Health at George Washington University.
"We also found that the mortality burden of this pandemic fell most heavily on younger people and those living in certain parts of the Americas," Simonsen said.
The 2009 pandemic was far from the worst such outbreak. In 1918, the Spanish influenza pandemic killed 50 million people, roughly 2% of the world population at that time.
Nevertheless, researchers said it was important for healthcare providers to understand the full impact of recent flu pandemics.
The relatively modest number of deaths estimated by the WHO prompted some to question whether the overall response to the 2009 outbreak was excessive. However, Simonsen and her colleagues argued that lab-confirmed influenza deaths would underestimate the broad reach of the illness.
"Many influenza-related deaths result from secondary bacterial infections or from exacerbation of preexisting chronic conditions, and are not recorded as related to influenza infection," authors wrote.
Among the findings that surprised researchers was the age and geographic distribution of deaths. Most of the people who died -- 62% to 85% -- were younger than age 65. Traditionally, seasonal influenza hits seniors the hardest.
[Updated 3:37 p.m. PST Nov. 26: The range in percentages for deaths under the age of 65 is due to a range of rates in the 20 sampled countries, as well as different statistical methods used to calculate global deaths.]
Researchers also calculated that flu-related deaths were 20 times greater in Central and South America than in European countries. That finding stood in sharp contrast to an earlier U.S. Centers for Disease Control and Prevention estimate that the heaviest mortality rates occurred in Africa and Southeast Asia, and that death rates in the Americas were lower.
The research was funded by the WHO and relied on viral illness and mortality data from 20 nations from 2005 to 2009. The sampled nations represented 35% of the world's population, and researchers then used statistical methods to calculate mortality rates for all nations.
The authors noted that their conclusions were limited by a lack of data from poor nations, and other factors.
"The true total mortality burden is likely to be even higher because deaths that occurred late in the winter of 2009-2010 and in later pandemic waves were missed in this analysis, and only pandemic influenza deaths that were recorded as respiratory deaths were included," authors wrote.
Researchers said that if deaths due to cardiovascular disease and other causes were included, the death toll might be as high as 400,000.