India may have grossly underestimated the might of 21st century's most aggressive pandemic.
A US Centers for Disease Control and Prevention (CDC), Atlanta, study with help from the All India Institute of Medical Sciences, (AIIMS), Delhi, to be published in the medical journal Lancet on Tuesday says the deaths caused by H1N1 pandemic flu in its first year (2009-10) could be 15 times higher than the number of laboratory-confirmed deaths previously reported to the World Health Organization.
During the pandemic, 18,500 laboratory-confirmed H1N1-deaths were reported worldwide from April, 2009, to August, 2010.
The new research indicates that the death toll was anywhere between 1.51 lakh and 5.75 lakh during the first year when the virus circulated in each country worldwide.
The results say that 80% of the deaths occurred in people younger than 65 years, contrary to seasonal influenza where most deaths occur among the elderly.
Additionally, the study suggests that 51% of the deaths may have occurred in south-east Asia and Africa, continents which are home to 38% of the world's population.
The scientists say, "China and India, where about a third of the world's population live have garnered little information about the burden of influenza."
India's age-adjusted respiratory and cardiovascular mortality rate associated with 2009 pandemic influenza H1N1 per 100 000 individuals stood at 4.1-6 per 100,000 population. Lead author Dr Fatimah S Dawood from the CDC said, "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 modeling 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".
It said, "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."
For the study, the CDC estimated the 12-month cumulative symptomatic attack rate (sAR) and multiplied it by the estimated symptomatic case fatality ratio (sCFR) from 17 sites in 13 countries — India, Bangladesh, Cambodia, Denmark, Germany, Kenya, Netherlands, New Zealand, Nicaragua, Peru, England, USA and Vietnam. Because the 2009 pandemic affected younger populations more than the senior citizens, the global burden in terms of years of life lost (YLL) was higher during this pandemic than it would be for a typical influenza season.
The study said, "Estimated YLL were 9707, 000 during the first 12 months of the pandemic. Southeast Asia was the region with the greatest YLL. Total YLL attributable to deaths associated with 2009 pandemic influenza A H1N1 was 3·4 times higher than if the age distribution of deaths had been similar to that during seasonal epidemics."
The study found that 65% of 2009 pandemic influenza A H1N1 deaths worldwide were in individuals aged 18-64 years (60% of global population), although the age distribution varied by region. Overall, 13% of respiratory deaths were in people older than 64 years (8% of global population).
"An additional 83,300 cardiovascular deaths associated with the 2009 pandemic influenza were estimated to have occurred in people older than 17 years, resulting in a total of 284,400 respiratory and cardiovascular deaths. Around 20% of these deaths occurred in people older than 64 years," it added.
Influenza pandemics are typically characterized by higher, but widely varying, number of deaths than seasonal epidemics. The emergence of pandemic influenza H1N1 in April, 2009, led WHO to request that countries report all laboratory-confirmed deaths associated with it.
"For the period up to August, 2010, 18,500 deaths associated with laboratory-confirmed 2009 pandemic influenza H1N1 have been reported. This number is likely to be an underestimate because diagnostic specimens are not always obtained from people who die with influenza and the viruses might no longer be detectable by the time of death in some people."