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The H1N1 flu pandemic of 2009-10 killed an estimated 284,500 people, according to Centers for Disease Control and Prevention (CDC) scientists — an estimate that is 15 times higher than the previously recorded official death toll.

The initial death count found that there were 18,500 H1N1 casualties between April 2009 and August 2010, but because this figure was based on laboratory-confirmed tests of victims, it likely vastly underestimated the true number of deaths. The recent CDC report puts the estimate at 151,700 to 575,400 deaths during the first year of the flu virus’ spread, with a median of about 284,500.

To calculate the new numbers, the CDC’s Dr. Fatimah S. Dawood and colleagues first estimated the percentage of people who contracted H1N1 in 12 countries, based on field data from individual communities in those countries; the researchers also estimated the percentage of people who died after contracting H1N1, and then looked at the overall estimate of deaths due to respiratory-tract infection in each country. Using this data, the researchers devised a statistical model to estimate the total global toll of H1N1.

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They found that the hardest hit regions were southeast Asia and Africa, where health-care providers and infrastructure may be lacking. According to the report, published in The Lancet Infectious Disease, 59% of all deaths may have occurred on these two continents, which are home to 38% of the world’s population.

“The study underscores the significant human toll of an influenza pandemic,” said lead author Dr. Fatimah S. Dawood of the CDC. “We hope that this work can be used not only to improve influenza disease burden modelling globally, but to improve the public health response during future pandemics in parts of the world that suffer more deaths, and to increase the public’s awareness of the importance of influenza prevention.”

Further, unlike the average flu, H1N1 tended to kill younger people: 80% of people who died from the pandemic flu were under the age of 65, and about a third of victims were healthy with no underlying health problems. “The number of deaths is not the only thing to look at in assessing the impact, but the huge loss of human potential,” Dawood told the Wall Street Journal. Because H1N1 killed so many young people, “more than three times the number of years [of life] were lost than are lost in a seasonal flu epidemic.”

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The new paper is only the first attempt to estimate the true global toll of the H1N1 pandemic. In a corresponding editorial, Dr. Cecile Viboud of the National Institutes of Health and Professor Lone Simonsen of George Washington University write that better estimates will arise as more data becomes available. They say the current study’s estimates are still lower than independent estimates of 2009 H1N1 deaths in Mexico, but that overall, the casualties of the H1N1 pandemic number fewer than those of other historic virus outbreaks.

“These results are likely to be refined as more studies from low-income and middle-income regions become available, particularly from China and India, where about a third of the world’s population live but where little information is available about the burden of influenza,” said Viboud in a statement. “More country-specific studies of disease burden are needed to elucidate the geographical determinants of influenza-related mortality and provide science-based rationale to refine global pandemic scenarios.”