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World Death Toll Of a Flu Pandemic Would Be 62 Million

"It is a huge, huge number," said study leader Christopher J.L. Murray. (Michel Euler - AP)

"The British colonial administration -- they were very good record-keepers," said Murray, who noted that India's contemporary death registries are less complete than ones from 1918.

The researchers compared the death rates during the 1918-1920 period with those in the three years before and after the pandemic. This gave an estimate of "excess mortality" during the flu years, which was assumed to be caused directly or indirectly by the virus. (Because men in countries fighting in World War I had elevated mortality in 1918, they were excluded from the calculation.) The extra deaths ranged from 0.2 percent of the population in Denmark to 7.8 percent in the Central Provinces and Berar region of India -- a 39-fold difference.

In the United States, they ranged from a low of 0.25 percent in Wisconsin to 1 percent in Colorado. (The best-known work of fiction about the pandemic, "Pale Horse, Pale Rider," is Katherine Anne Porter's account of her near-death experience during the Colorado outbreak.) Flu death rates varied greatly over short distances. Virginia's excess mortality, 0.47 percent, was well below Maryland's, 0.72 percent. Sweden's (0.66 percent) was three times Denmark's (0.2 percent).

Murray and his colleagues analyzed the death patterns and deduced that about half the variation from region to region was explained by differences in per capita income. For every 10 percent increase in income, a person's risk of dying during the pandemic fell 10 percent.

Why the poor were so vulnerable is unknown. It could have been that many were already ill with parasites or other illnesses or lacked micronutrients such as Vitamin A and zinc that are essential to immunity.

To estimate the effects of a modern Spanish flu, the researchers applied the 1918-1920 death rates to the current world population broken down by income, sex and age. They came up with a range of 51 million to 81 million deaths, with a median of 62 million.

Even though the world's population is three times what it was during the Spanish flu pandemic, the estimated mortality of a modern Spanish flu isn't three times what it was in 1918. That is mainly because per capita income is higher now -- and the higher the income, the lower the risk of dying of influenza.

The illness caused by the 1918 virus was largely untreatable. There were no antiviral drugs, no mechanical ventilators to help people breathe and no antibiotics to treat bacterial pneumonias that often set in after the viral infection. All are available now and would reduce the death toll, though some interventions would be in sort supply during a pandemic.

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