research team consisting of more than 60 collaborators in 26 countries has
estimated the global death toll from the 2009 outbreak of the H1N1 virus to be
10 times higher than the World Health Organisation's count, which was based on
laboratory-confirmed cases of this flu. The study, which appears online in PLOS
Medicine, suggests that the pandemic virus caused up to 203 000 respiratory
deaths around the world.
"This study confirms that the H1N1
virus killed many more people globally than originally believed," says
lead author Lone Simonsen, PhD, a research professor in the Department of
Global Health at the George Washington University School of Public Health and
Health Services. "We also found that the mortality burden of this pandemic
fell most heavily on younger people and those living in certain parts of the
The World Health Organisation (WHO), which
funded this study, had reports of 18 449 laboratory-confirmed deaths from the
2009 flu pandemic, but that is widely regarded as a low number because it is
based only on people with confirmed cases of H1N1. This study shows that the
actual death toll was much higher than the official count because most infected
people never got an H1N1 lab test.
The low number of confirmed deaths at least
initially led many to label the subsequent public health response as excessive.
Yet this study shows that the H1N1 virus, although not as lethal as the
infamous Spanish flu virus, still represented a formidable foe – killing many
more people around the globe than the original estimates.
Novel statistical procedure
In order to do this study, the team
obtained weekly virology data from the WHO and actual mortality data from 21
countries accounting for about 35% of the world's population. They used the
information to estimate the number of respiratory deaths, which often occur
when H1N1 gets into the lungs and causes pneumonia, in each of those 21
countries. They then used a novel statistical procedure to project those
results to the rest of the countries in the world.
In addition to deaths caused by respiratory
diseases, the H1N1 virus can also kill by exacerbating existing health
problems. And in fact, this team found that when the H1N1 deaths due to
cardiovascular disease and other causes are included, the 2009 pandemic toll
might be as high as 400 000.
The team discovered that an estimated 62%
to 85% of those who died in the 2009 pandemic were younger than age 65. That
high death toll for younger people is in marked contrast to that caused by
seasonal influenza, which mostly targets seniors.
The high casualty rate for people in their
prime translates to a bigger burden on individuals and society as younger
victims often mean more productive years of lost life, the authors said.
This study also showed a striking regional
pattern as H1N1 swept through certain countries, leaving a substantial number
of deaths in its wake. For example, the researchers found an almost 20-fold
higher mortality rate in some countries in the Americas with Mexico, Argentina
and Brazil showing the highest respiratory death rates in the world. In
contrast, the toll was far lower in New Zealand, Australia and most parts of
The geographical mortality pattern in this
study differs markedly from a 2012 study by the Centres for Disease Control and
Prevention that estimated the impact due to the H1N1 pandemic before 2009
national vital statistics became widely available. Although that study's
results led to a similar global mortality estimate, the CDC researchers came up
with a very different regional map of the burden, with very heavy death counts
in Africa and in Southeast Asia and lower death rates in the Americas and
This study failed to find the same high
death toll in a few measured countries in Africa and Southeast Asia but
Simonsen says that very few data exist for what really happened during the
pandemic in these regions. She says additional studies will need to be done in
order to understand the course of the virus as it spread through low-income
parts of the world. In addition, researchers must continue to study the
pandemic in order to find out why some countries were so hard hit and others
were mostly spared.
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. The 1918 Spanish influenza
pandemic, for example, killed approximately 2 percent 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, Simonsen says.