By Laura Cowen
The true number of deaths from the 2009 H1N1 "swine flu" pandemic may be 15 times higher than the number confirmed by laboratory testing and previously reported to the World Health Organization (WHO), study findings indicate.
A total of 18,500 laboratory-confirmed deaths caused by the 2009 pandemic influenza A H1N1 were reported worldwide for the period April 2009 to August 2010.
However, using statistical modeling, an international team of researchers estimates that the actual number of deaths from associated respiratory illness was between 105,700 and 295,600.
Furthermore, adding cardiovascular deaths associated with 2009 pandemic influenza A H1N1 among people older than 17 years increases the mortality burden to 151,700‑575,400 deaths.
Consistent with previous reports, most deaths occurred in people aged 18‑64 years.
To arrive at their estimates, Fatimah Dawood (Centers for Disease Control and Prevention, Atlanta, Georgia, USA) developed an age-stratified probabilistic model that estimates mortality rates using data on virus-associated symptomatic attack rates from 12 countries and symptomatic case-fatality ratios (sCFR) from five high-income countries. They accounted for between-country differences in mortality risk, by developing multiplier factors based on respiratory disease mortality before the pandemic.
Writing in The Lancet Infectious Diseases, the researchers note that a disproportionate number of total deaths from cardiovascular and respiratory diseases (51%) was estimated to have occurred in the African and southeast-Asian regions, where 38% of the world's population live and where data for influenza incidence are scarce.
This finding highlights the need to expand production and improve delivery of influenza vaccines to Africa and southeast Asia, the authors remark.
Dawood said in a press statement: "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 researchers concede that their model was limited by a lack of globally representative data on symptomatic attack and case-fatality rates.
"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," they say.
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