AsianScientist (Jul. 9, 2012) – And they say numbers don’t lie. A new report by scientists from the Centers for Disease Control and Prevention (CDC) in the United States estimates that the 2009 influenza A H1N1 pandemic may have killed up to half a million people around the world.
These new figures are dramatically higher than the 18,500 laboratory-confirmed deaths reported by the World Health Organization (WHO) between April 2009, when the new strain of virus was identified in Mexico, and August 2010.
The new report was published on June 26 in The Lancet Infectious Diseases medical journal.
To get a better estimate of the actual number of H1N1-related deaths, CDC researchers created a probabilistic model to estimate global mortality and number of years of life lost (YLL) in three different age groups.
They drew on data from a total of 12 high, middle and low-income countries to adjust for country differences in socioeconomic factors and health care.
The model also included WHO data on lower respiratory tract infection mortality rates before and during the pandemic. This mortality ratio, called the respiratory mortality multiplier (RMM), accounted for influenza-related ‘excess mortality’ during the pandemic period.
Based on this model, the estimated number of deaths associated with the pandemic flu was 201,200 for respiratory diseases with an additional 83,300 for cardiovascular diseases in the first year of virus circulation. Taking into account other factors, the researchers estimated that the combined toll could have been in the range of 151,700 to 575,400 deaths.
“This is a better approximation of the number of deaths that occurred,” said Dr. Marc-Alain Widdowson from the influenza division at the CDC who was a senior author on the paper.
Widdowson also noted that 80 percent of deaths were in the under-65 age group, which is very different from seasonal influenza, where the vast majority of deaths are in the over-65 age group.
As a rule, the number of laboratory-confirmed deaths from any disease outbreak is assumed to underestimate the actual deaths. Underreporting in middle and low-income countries could be due to the lack of diagnostic and treatment facilities in these regions, which mean that fewer patients are tested for the flu and fewer deaths are attributed to the flu.
Hence, the authors believe that the flu may have taken its greatest toll in Southeast Asia and Africa, home to 38 percent of the world’s population, where 51 percent of the deaths may have occurred.
In the earlier WHO report, less than 12 percent of the global disease burden was reported in Africa and Southeast Asia.
“Although no estimates of symptomatic case fatality ratios (sCFRs) were available from Africa and Southeast Asia, a disproportionate number of estimated pandemic deaths might have occurred in these regions. Therefore, efforts to prevent influenza need to effectively target these regions in future pandemics,” the authors wrote.
But Dr. Cecile Viboud of the National Institutes of Health and Lone Simonsen of George Washington University, in an accompanying comment, noted that most estimates of mortality were derived from high-income countries that routinely collect data of the number of people who get the flu and die from it.
They also cautioned that the modeling method used to calculate the deaths has not been used before and contained uncertainties.
The lack of accuracy in the data from low and medium-income countries accounted for the wide variation in the estimate, they explained.
Independent estimates of pandemic mortality based on solid vital statistics fell within the CDC predicted range for the US and Bangladesh, but were far higher in the case of Mexico. By contrast, independent estimates for Japan and Singapore were lower than CDC estimates.
“Multipliers 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,” the authors wrote in the editorial.
Within a month of its discovery, the virus had spread to 43 countries, with 12,515 reported cases and 91 related deaths worldwide.
Scientists working to sequence the new human influenza A H1N1 virus then discovered that it had evolved over nearly two decades through genetic reassortment of avian, human (H3N2), and classic swine (H1N1) strains, further combined with a Eurasian swine flu strain.
By August 2010, the WHO declared the worst to be over as worldwide flu activity had returned to typical seasonal patterns.
The exact mortality figures may be debated for several more years to come, but researchers will all agree on one thing – the severity of the 2009 flu pandemic was nowhere near the 1918 Spanish flu that caused tens of millions of deaths worldwide.
With the world’s population tipping over seven billion, combined with the ease of travel, researchers at the CDC and WHO can only hope that the next flu pandemic will eventually come out short in numbers, both official and otherwise.
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Copyright: Asian Scientist Magazine.
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