A new study by over 60 researchers from 26 countries suggests that the number of people who died in the 2009 global H1N1 "swine flu" outbreak is much higher than official figures show.
Writing in PLOS Medicine, they estimate the total respiratory deaths worldwide to be up to 203,000, some 10 times higher than the World Health Organization (WHO) official total, which is based on lab-confirmed cases.
When they added deaths by other causes exacerbated by H1N1, the figure was nearly double this amount, or over 20 times higher than the WHO lab-confirmed cases.
Commenting on the WHO-funded study, lead author Lone Simonsen, a research professor in the Department of Global Health at the George Washington University School of Public Health and Health Services in the US, says:
"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."
Based only on lab-confirmed cases, the WHO official figures report that just under 18,500 people died in the 2009 flu pandemic.
This total was already considered to be much lower than actual numbers because not all cases are lab-confirmed in a flu outbreak.
The new study's findings are important because they help us to consider whether the public health response to the outbreak was excessive or not. Based on the official figures, some might suggest it was, but based on these new estimates, that may no longer be the case.
Although nowhere near as devastating as the 1918 Spanish flu, which killed tens of millions of people, a death count in the hundreds of thousands still represents a formidable foe.
Study used respiratory deaths data from 21 countries
For the study, the team used weekly virology data kept by the WHO and information on actual deaths directly from 21 countries, representing aprroximately 35% of the world's population.
From this, they could estimate numbers of respiratory deaths in those countries (H1N1 causes respiratory death when it gets into the lungs and causes pneumonia).
They used the estimates from the 21 countries with a new statistical method to arrive at a global figure, yielding a global death count of 203,000 respiratory deaths due to the 2009 H1N1 outbreak.
However, when they took into account the fact that H1N1 can also cause deaths by making existing conditions - such as heart disease - worse, the team discovered the 2009 death toll could be as high as 400,000.
The researchers also estimate that between 62-85% of deaths in the 2009 H1N1 swine flu pandemic were among people under 65 years of age - in marked contrast to seasonal flu epidemics where the highest proportion of deaths occurs mainly among seniors.
Regional variations differ from previous estimate
In terms of individual countries, there were also some stark contrasts. For instance in the Americas, countries including Mexico, Argentina and Brazil suffered a swine flu casualty rate some 20 times higher than other countries - the highest respiratory death rates in the world.
Parts of Europe, plus Australia and New Zealand, in contrast, had much lower rates of deaths.
The regional variation that this study suggests is quite different from the 2009 pattern of swine flu deaths reported by the US Centers for Disease Control and Prevention (CDC) in 2012.
The CDC estimated that the death count was very heavy in Africa and Southeast Asia and lower in the Americas and Europe.
The researchers in this new study could not find figures to substantiate the CDC estimates in Africa and Southeast Asia, but Prof. Simonsen says it is not easy to investigate these regions because there is insufficient data to make reliable estimates of what really happened.
She calls for further studies to look more closely at how the virus spread in the poorer nations. Plus, she says it is important to continue studying the pandemic to find out why some countries suffered much worse than others.