But looking at the total number of deaths is a misleading way to analyze the toll of the 2009 outbreak, said the first author of the paper, Prof. Lone Simonsen of the school of public health at George Washington University in Washington, D.C.
That's because where about 80 per cent of people who die from seasonal flu are 65 and older, between 62 per cent and 85 per cent of the people who succumbed to pandemic H1N1 were younger than 65.
"It's apples and oranges because these pandemic deaths are much younger than the seasonal (flu) deaths," said Simonsen, who has extensively studied the death tolls of known flu pandemics.
"One should be dealing with years of life lost or something to try to really understand the texture as much as the cut."
When one uses years of life lost as a measuring stick against which to assess the pandemic, its severity is in line with the 1968 Hong Kong flu, the pandemic which preceded the 2009 outbreak, she said.
Michael Osterholm, an infectious diseases expert at the University of Minnesota, agreed years of life lost is a better way to gauge the impact of the 2009 pandemic.
"A flu death is not just a flu death," said Osterholm, who is the director of the university's Center for Infectious Diseases Research and Policy.
"While we'd like to prevent all flu deaths, a flu death in a person who has a number of co-morbidities (other diseases) at age 78 is very different than that of a young, pregnant woman — otherwise healthy — at age 22."
Osterholm was not involved in this study, which was published Tuesday in the journal PLoS Medicine, a publication of the U.S. Public Library of Science. The authors were from the U.S., the Netherlands, Britain and from the World Health Organization in Geneva.
Osterholm noted that the mean age of people who died in the H1N1 outbreak was 40, at a time when life expectancy was 79 years.
Roughly a million people are believed to have died in the 1968 pandemic. But in that event, the mean age of people who died was 62, at a time when the average life expectancy was 71 years.
"Number of deaths is a misleading indicator of the severity of influenza," Osterholm insisted.
"It's early, premature deaths that we should be in a position of preventing. And this is what this pandemic pointed out — that this was not seasonal flu."
Another key finding of the study was that some parts of the world were hit far harder than others, with 20 times more deaths in the Americas than in Europe.
While early reports out of Mexico — where the virus was first spotted — raised alarm that the outbreak might be very severe, some parts of the world saw such mild activity that people questioned the decision to declare the outbreak a pandemic.
France ended up sending back most of the pandemic vaccine it ordered because few people agreed to be vaccinated. And some politicians at the Council of Europe were heavily critical of the WHO's actions, accusing the agency of setting off a false alarm.
Simonsen said there were huge variations in how regions and even countries within regions were hit. For instance, while the pandemic's severity was highest in the Americas, Chile weathered the storm much better than neighbouring Argentina. Likewise, Romania was badly hit — though most of the rest of Europe saw mainly mild illness.
It's not clear why there was such variability, said Simonsen, who noted that the fact that countries with similar socio-economic status and health-care systems had markedly different experiences suggests one cannot put it down to the calibre of medical care or the basic health of the populace.
Another thing the paper shows is the complexity of calculating deaths in an event like a flu pandemic.
Influenza doesn't always — or often — kill directly. In many fatal cases, flu sufferers will go on to develop pneumonia. If they die of pneumonia or of a heart attack brought on by their illness, their death certificate often does not indicate that influenza started the cascade that led to death.
So researchers like Simonsen and her co-authors used indirect methods to measure fatalities, looking for unusual peaks in deaths — called excess mortality — during flu season or a pandemic.
For this study, Simonsen and her co-authors looked at mortality data from 20 countries representing 35 per cent of the world's population for the period from 2005 to 2009. Using that data, they built a mathematical model to calculate global figures.
They suggested between 123,000 and 203,000 people died from pandemic flu in the final nine months of 2009 — about 10 times more than the 18,631 deaths that the WHO confirmed.
Their model suggests that for the four years preceding the pandemic, between 148,000 and 249,000 people a year died from seasonal flu, on average.
But the authors said the paper likely substantially underestimates the total death toll for the 2009 pandemic. For one thing, Simonsen said, people continued to die from the pandemic strain into 2010 and even later.
And the team wasn't able to get mortality data for many parts of the world. They suggested the true death toll may have been in the 300,000 to 400,000 range.
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