Friday, June 23, 2006

Odds and Outs of a Bird Flu Pandemic

A little calibration --- and some Hold'em analogies --- may help to
illustrate your personal odds of dying in an 1918-level pandemic. To focus the discussion, let's take the US Government's scenario where there are two million pandemic deaths in the US. This is a horrifying number, but many people will find it less frightening once the two million is put in perspective.

First, two million is only about 2/3 of one percent of the population. Thus, if you are a random US resident, and if the government's worst case scenario unfolds, you have only about a probability p=0.0066 of dying in the pandemic. In the familiar terms of Hold'em poker, this is just a bit more than the probability of being dealt pocket Aces (1/221=0.00452) and a bit smaller than the probability of being dealt pocket Aces or Kings (2/221=0.00904).

Getting Additional Outs

Would you like to cut your own personal p down from p=0.0066 to say a fifth of that? That would make your personal p about 0.0013, or a bit more than a one-in-a-thousand shot. This seems to be a practical goal that is well worth the price of a little a planning.

The Non-controversial Basics

* First, everybody is better off if person-to-person contact is minimized.
* If you are quarantined then for heaven's sake, stay quarantined.

Schools should have a very low threshold for closing, and rational parents should have an even lower threshold for keeping their kids out of school. If you have toyed with the idea of home schooling, this is the time to get serious. Staying alive does depend on luck, but you tilt the odds in your favor by minimizing exposure ("social distancing") and by paying close attention to hygiene, especially washing your hands and disinfecting items that you bring into your house.

How about Masks?

The benefit of using a N95 mask is a little more speculative. In aggregate, they may not help much because very few people will take the trouble to train themselves to use the masks appropriately. The must be correctly fitted, and --- most important --- they must be removed in a way that avoids accidental post-use contamination.

Also, simple physics suggest that masks have a high hurdle to clear. A single virus and an N95 mask have about the same relationship as a golf ball and a tuna net. The single virus goes through easily. Where the masks help is in stopping those big wads of virus that have the analogous size of a softball --- or even a basketball. These wads can be stopped, and it is worthwhile to stop them.

There is even some empirical evidence that masks can help. In the SARS epidemic N95 masks made a useful difference. They reduced caregiver infection from 6% (per shift!) to 1% (per shift!). Here of course, the medical personnel were trained in the proper use of the masks.

Finally, there are two hidden benefits of the use of masks. First, the wearer will touch his face less often and thus cut down on one of the principal pathways of infection. Second, a mask wearer is likely to be accorded more than the usual "social distance," since --- at the beginning at least --- it is likely to be assumed that the wearer is ill.

Anti-Virals: The Major Source of Outs

The Roche anti-viral Tamiflu is now regarded as the best available tool for both the prevention of bird flu infection and the treatment of infected persons. When the history of the 21st Century flu pandemic is written, there will be many volumes that will treat the science, politics, and ethical storms that surround Tamiflu and other anti-virals. In subsequent posts I will explore several senarios of Tamiflu-economics.


Blogger gs said...

most people seem to assume that the CFR (case fatality rate) would likely go down from actually 55% to 2% as in 1918 in USA or even lower as in 1957,1968. I can't see this.

6:16 AM  

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