A large, collaborative study concluded that the global death toll from the 2009 pandemic of the H1N1 virus reached up to 203,000. This tally was based on laboratory-confirmed influenza cases and is almost 10 times the number of respiratory deaths that the World Health Organization (WHO) had originally calulated.

"This study confirms that the H1N1 virus killed many more people globally than originally believed," lead author Lone Simonsen, a research professor in the Department of Global Health at the George Washington University School of Public Health and Health Services, remarked in a press release. "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."

The large difference in death toll estimates hinges on the fact that WHO based their calculations solely on confirmed cases of H1N1. The current study took into account people who were infected but never tested for the H1N1 strain of influenza. The readjusted death toll confirms a view held by many experts that the H1N1 outbreak of 2009 was more lethal and dangerous than previously shown.

To gain a clearer picture about the pandemic, researchers calculated the number of respiratory deaths in 21 countries, which accounted for approximately 35 percent of the world’s population. From that figure they were able to statistically calculate the extent that the virus hit the rest of the countries across the globe.

One unexpected pattern that emerged was a spike in deaths in Mexico, Argentina, and Brazil, which had the highest respiratory death rate compared to the rest of the world — up to 20 times higher in some cases. New Zealand, Australia, and a majority of Europe fared much better on the other hand. This finding greatly differs with a 2012 study conducted by the Centers for Disease Control and Prevention (CDC) that found a much higher concentration of victims in Africa and Southeast Asia, which they concluded was a reflection of limited access to prevention and treatment resources. The CDC’s estimated death toll, however, was much closer to the current study’s, and more than 15 times higher than WHO’s estimate.

Simonses explained that this discrepancy in death counts could be due to the less developed, low-income regions unable to accurately ascertain the outcome of the pandemic. Further studies, she added, need to delve into how these regions of the world were hit by the pandemic and, importantly, why some regions suffered significantly more casualties than others. The researchers also noted that the updated death toll doesn’t include people who had other health issues, such as cardiovascular disease, which were made unimaginably worse by the H1N1 virus. Taking these cases into account would push the death toll up to 400,000, the researchers estimate.

Another prominent number that emerged from the study was the disproportionately high death toll for younger people; 62 to 85 percent of those who died from an H1N1 infection were under the age of 65. Even though the pandemic mortality estimate was similar in magnitude to that of seasonal influenza, the authors commented in the study, a marked shift toward mortality among persons below 65 years of age occurred, "so that many more life-years were lost".

"Going forward ... countries and existing networks should partner to collaboratively and rapidly assess the severity of future pandemic threats,"  the researchers conclude in their paper.