Scientists now estimate that about 280,000 people died in the flu pandemic that swept the globe in 2009 and 2010, more than 15 times the toll that had been officially recorded by laboratory tests of victims.

The big increase in estimated deaths from the H1N1 virus isn't a surprise, experts said. In any pandemic, the initial count typically is based on lab-confirmed figures that tend to significantly underestimate the true number of fatalities. In the case of the H1N1 pandemic, there were 18,500 lab-confirmed deaths, while a more comprehensiveestimate suggests that anywhere from 151,700 to 575,400 people may have died. The 280,000 figure is a rough median estimate for that range.

The new study, published in the Lancet, is the first effort to calculate the global impact of the H1N1 virus, known at the time as swine flu. The contagion was triggered by a recombination of bird, pig and human viruses and was the first flu pandemic of the 21st century. The pandemic officially ran from June 2009 to August 2010.

H1N1 caused havoc around the world, sickening or killing hundreds of thousands of people. It prompted several national public-health emergencies and pushed countries to stockpile vast quantities of vaccine and antiviral drugs.

The latest research confirmed earlier observations that 80% of the pandemic deaths occurred in people younger than 65 years old. About 30% of the victims in that age group were healthy with no underlying risk factors. That is a marked difference from seasonal flu, which mainly claims the elderly and the frail.

The study also found that nearly 60% of the fatalities were likely to have occurred in Southeast Asia and Africa, home to 38% of the global population. The highest mortality rates were in Africa, which suffers from a poor health infrastructure and lack of trained medical personnel.

'The number of deaths is not the only thing to look at in assessing the impact, but the huge loss of human potential,' said Fatimah Dawood, an epidemiologist and pediatrician at the U.S. Centers for Disease Control and Prevention in Atlanta, who was lead author of the paper. Because of the many younger H1N1 victims, she added, 'more than three times the number of years [of life] were lost than are lost in a seasonal flu epidemic.'