“Swine flu” deaths may be fifteen times higher than previously reported

The number of deaths from the 2009 H1N1 (“swine flu”) pandemic may be fifteen times higher than the number of laboratory-confirmed deaths previously reported by countries to the World Health Organization (WHO), according to an Article published Online First in The Lancet Infectious Diseases. While estimates of 2009 H1N1-associated deaths have been derived for some countries such as the United States, these are the first estimates of the worldwide number of deaths caused by the pandemic.

During the pandemic, 18,500 laboratory-confirmed H1N1-deaths were reported worldwide from April 2009 to August 2010.  However, as a general rule, the number of lab-confirmed flu deaths is known to be significantly lower than the number of flu deaths that actually occur, and the new research indicates that an estimated 151,700 to 575,400 people died as a result of having contracted 2009 H1N1 during the first year that the virus circulated in each country worldwide.

The results suggest that 80% of the deaths occurred in people younger than 65 years, contrary to seasonal influenza where most deaths occur among the elderly.  Because the 2009 pandemic affected younger populations more than older populations, the global burden in terms of years of life lost was higher during this pandemic than it would be for a typical influenza season.  Additionally, the study suggests that 51% of the deaths may have occurred in southeast Asia and Africa, continents which are home to 38% of the world’s population, with the highest mortality rates occurring in Africa.

According to lead author Dr Fatimah S. Dawood of the US Government’s Centers for Disease Control and Prevention: “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 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”.

To arrive at their estimates, a global team of researchers developed a new model that used influenza-specific data from 12 low-, middle-, and high-income countries. The authors hypothesised that risk of death from flu is higher in some countries than others, and used WHO data on lower respiratory tract mortality in different countries to account for these differences.

The new estimates of flu deaths are based only on data from countries that hold information on the numbers of people who developed flu symptoms as well as the numbers of deaths among flu cases during the 2009 H1N1 pandemic.  However, shortcomings in the availability of high-quality data for most countries affected by the pandemic may affect the accuracy of the new estimates, with the authors pointing out that the lack of data is particularly pronounced for low-and middle-income countries.

According to the authors, “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 linked Comment, Dr Cecile Viboud of the National Institutes of Health, Maryland, USA, and Professor Lone Simonsen of George Washington University, Washington DC, USA, find that the new estimates compare favourably with independent estimates of 2009 H1N1 mortality in some countries, being broadly in agreement with existing estimates for Bangladesh and the USA.  However, the new estimates of mortality rates are significantly lower than independent estimates of 2009 H1N1 deaths in Mexico. 

According to Dr Viboud, “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.”