According to the World Health Organization (WHO), the H1N1 (“swine flu”) pandemic killed 18,500 people from April 2009 to August 2010. However, a new study reveals that this figure may actually be fifteen times higher.

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Up to 575,400 people are thought to have died after contracting swine flu.

The study, published online in The Lancet Infectious Diseases, notes that the number of lab-confirmed flu deaths is known to be considerably lower than the actual number of flu deaths that occurred. The researchers estimate that 151,700 to 575,400 people died due to contracting H1N1 in 2009, and that 80% of the deaths occurred in people under 65 years of age.

Furthermore, their results indicate that 59% of the deaths may have occurred in Africa and southeast Asia, with the highest mortality rates occurring in Africa. These continents are home to 38% of the global population.

Lead author of the study Dr. Fatimah S. Dawood of the US Government’s Centers for Disease Control and Prevention, explained:

“The study underscores the significant human toll of an influenza pandemic. We hope that this work can be used not only to improve influenza disease burden modeling 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.”

The team used a new model that used influenza-specific data from 12 low-, middle-, and high- income countries to arrive at their estimates. The researchers believed that the chances of dying from flu is higher in some countries than others. As a result, the team used WHO data on lower respiratory tract mortality in different countries in order to account for these differences.

The new estimates were based only on data from countries that hold information on the number of people who developed flu symptoms, and the number of flu-related deaths that occurred during the 2009 H1N1 pandemic.

The researchers highlight that the accuracy of their estimates may be affected by shortcomings in the availability of high-quality data for the majority of countries, particularly for low- and middle-income countries, affected by the pandemic.

The researchers said: “Continued efforts to strengthen influenza surveillance worldwide, particularly for influenza-associated mortality, are needed both to guide seasonal influenza prevention strategies and to build influenza surveillance systems to provide better and more timely and globally representative data for influenza-associated mortality during future pandemics.”

In a joint comment, Dr. Cecile Viboud of the National Institutes of Health, Maryland, USA, and Professor Lone Simonsen of George Washington University, Washington DC, USA, said that although the new estimates may be accurate for some countries, the new estimates of mortality rates are significantly lower than independent estimates of 2009 H1N1 deaths in Mexico.

Dr Viboud explained:

“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. 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.”