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What are the odds that H1N1 will kill you?

July 14, 2009 |  4:01 pm

It’s the one thing everyone really wants to know about the H1N1 pandemic flu: How deadly is it?

By adding up the number of confirmed deaths (429, according to the latest World Health Organization figures) and dividing it by the number of confirmed cases (the WHO says it’s 94,512), the simple answer is that the fatality rate is just below 0.5%. That’s in line with a bad year for the seasonal flu. But is it accurate?

Not necessarily, according to a group of experts from the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial College London. In a paper just published online by the British Medical Journal, they describe the reasons why that easy estimate could miss the mark.

For instance, once somebody comes down with the flu, it takes several days or weeks to either die or recover. At any point in time, the number of people known to have become infected is always bigger than the number of people for whom outcomes are known. That means the calculated fatality rate will be too low, and it will grow as the outbreak progresses.

This is exactly what happened in 2003, when severe acute respiratory syndrome spread through Asia, Europe and North America. As the fatality rate rose, people feared the virus had mutated into a deadlier form. If the numbers show H1N1 becoming more lethal when the traditional flu season arrives in the fall, how will public health officials know whether the virus is actually more dangerous? Epidemiologists should take into account the lag time between the onset of flu symptoms and the outcome of infection, the researchers write.

On the other hand, the fact that people with severe symptoms are more likely to interact with the medical system means the calculated fatality rate probably overstates the lethality of the outbreak. “If we take mild unreported cases into account, the true case fatality ratios could therefore be considerably lower and comparable to that for seasonal influenza,” the researchers write.

To overcome this source of bias, they suggest close monitoring of at least 1,100 patients from the initial stage of infection to get a full picture of the proportion that requires hospitalization -- and the proportion of hospitalized patients that succumbs.

Getting an accurate assessment of H1N1’s fatality rate will be crucial in determining the need for school closures and other measures aimed at preventing the spread of the virus, the researchers said.

-- Karen Kaplan

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