H1N1 death projections: doing the math
By Laura Blue, courtesy of TIME
A new headline-grabbing report from the White House claims that swine flu could plausibly infect up to 50% of Americans, causing flu symptoms among some 60 to 120 million of them, and leading to as many as 1.8 million hospitalizations and 30,000 -90,000 deaths.
Where, exactly, do numbers like these come from? The new report was put together by the President’s Council of Advisors on Science and Technology. It turns out the predictions are based on just a couple key facts:
- The virus seems to be transmitted from person to person at the same rate as in previous flu pandemics — a rate that’s much higher than that of the regular seasonal flu. Rapid transmission suggests that the total number of infections could be very high.
- The death rate for people who catch H1N1 seems about the same as that for seasonal flu. The White House advisors estimate that, so far, between 1 in 1,000 and 3 in 1,000 people who have needed medical help then end up dying. Assuming that this normal death rate continues during flu season, the total number of deaths is projected to be much higher than normal because of the higher number of infections.
And that’s basically it. The Council’s report notes prominently and often that, even though the up-to-50%-infected scenario is plausible, it is by no means certain. That’s because both of the basic facts above — the infection rate and the case fatality rate — are still a little fuzzy. They’re hard to measure in the first place, and it’s not totally clear whether they’ll change as the pandemic progresses.
So why all the fuss if the estimates are still murky? As Homeland Security head Janet Napolitano put it yesterday in a statement: "It is not possible to predict how the 2009-H1N1 influenza virus or the upcoming influenza season will play out, but it is best that we plan and prepare for a resurgence of H1N1 flu." Things may not develop the way the White House advisors suggest, in other words, but given available evidence it’s still a fine idea to brace ourselves.
There’s one other very good reason that this year’s flu pandemic has experts unnerved. Seasonal flu typically kills only the elderly, with 90% of victims 65 and older, according to the new White House report. (That’s one of the reasons we don’t consider seasonal flu a very serious disease, even though it kills thousands of Americans very year; the public just doesn’t get too upset over a 92-year-old passing quietly in the night.) But the pandemic flu has hit young people and working-age adults unusually hard, just as the deadly 1918 flu did. So far 83% of U.S. deaths and 71% of hospitalizations have occurred among people aged 5 to 64. That means that every death is responsible for more years of life lost.
