Deaths Caused by the 2009 Flu Pandemic Greatly Exceeded WHO Estimates

A new estimate suggests that the global death toll from the 2009 outbreak of the H1N1 virus to be 10 times higher than the World Health Organization’s reckoning, which was based only on laboratory-confirmed cases.

A team of researchers comprising more than 60 collaborators in 26 countries analyzed weekly virology data from the World Health Organization (WHO) FluNet database and national influenza centers to identify influenza active periods, and obtained weekly national underlying cause-of-death time series for 2005–2009 from collaborators in more than 20 countries (35% of the world's population).

The investigators employed a multivariate linear regression model to measure the numbers and rates of pandemic influenza respiratory deaths in each of these countries. Then, in the second stage of their analysis, they used a multiple imputation model that took into account country-specific geographical, economic, and health indicators to project the single-country estimates to all world countries.

The WHO reported 18,631 laboratory-confirmed pandemic deaths, but the total pandemic mortality burden was substantially higher. The results of the current study yielded an estimate that between 123,000 and 203,000 pandemic influenza respiratory deaths occurred globally from April 1, 2009, through December 31, 2009. Most of these deaths (62%–85%) occurred in people younger than 65 years old. There was a striking regional heterogeneity in deaths, with up to 20-fold higher mortality in Central and South American countries than in European countries. The model attributed 148,000–249,000 respiratory deaths to influenza in an average prepandemic season. Notably, only 19% of these deaths occurred in people younger than 65 years old.

“This study confirms that the H1N1 virus killed many more people globally than originally believed,” said first author Dr. Lone Simonsen, professor of global health epidemiology at George Washington University (Washington DC, USA). “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.”

“Whenever a new influenza virus emerges the ensuing outbreak can represent a crisis—with rapidly spreading illness and death that spreads from country to country,” said Dr. Simonsen. “The 1918 Spanish influenza pandemic, for example, killed approximately 2% of the world population at the time or a staggering 50 million. Although the H1N1 flu did not come close to causing that high casualty rate, understanding the global impact of such a pandemic remains vitally important in order to plan and prepare for the next time a pandemic virus emerges.”

The study was published in the November 26, 2013, online edition of the journal PLOS Medicine.

Related Links:
George Washington University
World Health Organization (WHO) FluNet database