Wednesday, April 29, 2009

Flu risk in perspective

The current swine flu outbreak is frequently compared to the 1918 Spanish flu pandemic. An article (that I found via Boing Boing) by Stephen Hume in the Vancouver Sun puts the individual risk of the Spanish flue in perspective:

It's estimated that about 28 per cent of Canadians and Americans contracted the Spanish flu. Worldwide, an estimated 2.5 per cent of the sick died of complications, which made the pandemic one of the most lethal flu outbreaks in recorded history. Certainly it was one that imprinted itself upon human consciousness for several generations.

But there's another way to look at those statistics. You might observe, for example, that they mean that even during the worst ravages of the 1918 flu, 97.5 per cent of those infected survived and recovered. Or that 72 per cent of the population -- even in the absence of the sophisticated public health planning and infrastructure that Canada and the U.S. have since built -- was not infected during the pandemic.

A pandemic on the scale of the 1918 flu is clearly a public health threat. With the same rates of infection and mortality on the current U.S. population, this would imply 300M * 0.72 * 0.025 = 5.4 million deaths. Yet the risk to the average individual is 0.72 * .025 = .018 or 1.8%. The risk of the current swine flu outbreak is not known, and neither Hume no I mean to imply that the risks are comparable.

In a typical year flu kills about 30,000 people in the United States, mostly the elderly.


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