In in the linking page, I asserted that I believe that there is a 20% chance of an H5N1 pandemic beginning in 2006. The way I come to this number is shamefully ragged, but for my own planning I need to come up with some number. I expect to change my estimate and to change my method, but, in the meanwhile, I will share what I've got.
First, influenza pandemics have always been with us, and the informal base rate is "several times" a century. This makes the assertion that "there will be a pandemic" rather banal.
Still, the current situation is far from banal, so something is different. The difference of course is the emergence of the H5N1 avian strain that appears never to have been in the human population.
All of these are a priori factors that suggest that the historical base rate (of say 1/30) needs to be multiplied by some factor. What factor? Now, I start trying to look around the corners. You may not like my modeling. I don't love it myself.
First, let's note that many national governments are taking serious actions, even though many remember that the "swine flu" pandemic fizzled out in the '70s and became a political embarrassment to Gerald Ford and to many public health officials. Politicians will also recall that the pandemic of 1968 (700K dead) was mild in contrast to what world health warned about at the time.
Nevertheless, governments are now acting on expert advice --- and I don't think they would do that if they thought that they were facing a probability as small as 1/10. My inference is that the most informed subject matter experts must have "conveyed" at least a 1/5 probability to these politicians to get them off the dime. If this persuasion happened in one or two countries, it could just be politics --- but given the short planning horizons of politicians there was lots of independent judgment that came to the conclusion that "this thing is going to happen --- and happen pretty soon."
So, I'm starting to think 1/5 looks reasonable --- perhaps a shade conservative. Certainly this is not hard science, and it would be easy to make fun of this reasoning. Still, I am trying to be honest about something that I think is important, and I am not going to pretend that there is anything more scientific to my reasoning than there really is. As Dean Foster points out, I'm only looking at a factor of 6 over the base rate (though I am positing a higher lethality than "most pandemics").
There is a further --- unscientific but sensible --- reason for suggesting 1/5 as a working hypothesis. With a 1/5 assumption the risks are high enough that you can take the issue very seriously and make certain preparations, but you have not driven yourself in to a "survivalist" corner. Naturally, as new data arrives, the survivalist options start to look more attractive, but let's take one step at at time. When the WHO asserts that we have entered Stage 4, "survivalist" thoughts should be welcome.
I believe that a 1/5 geometric arrival rate may be about as good as a street-smart gambler can guess. I don't think the rate could be anywhere near as low as 1/10, and, even if we move quickly to Stage 5, one really can't say that a hypothesis of 1/2 would have been much smarter than the candidate 1/5. In fact, 1/5 has several useful features:
This is an attempt to reason in the face of great ignorance, but I believe that anyone's informed attempt provides wiser guidance than simply throwing up one's hands and saying we don't know. Certainly, we don't KNOW, but by taking 1/5 seriously we make some progress. Now to me there are some bets seem attractive and some that seem unattractive.
Foreign Affairs 2005: This is the single piece that sophisticated policy makers are most likely to have read. Two quotes:
Its the second quote that makes me believe that the H5N1 pandemic is essentially certain. There is simply no stopping a mixing bowl of 500 million under regulated pigs.
Strategies for Mitigating a Pandemic --- An informative scientific article. Probability modeling, but without explicit definition of the models used. Aims to give advice about effective deployment of anti-virals and vaccines. Asserts an approximate 1/3 law: infections come 1/3 from household, 1/3 from work/school, and 1/3 from elsewhere. This is good news for small families without school age children and who can work from home.
Pandemic Mitigation and Self-Sheltering ---Non-academic think piece with references that engages the efficacy of minimized social contact, school closing, etc.
Fidelity Investments --- Asset Returns During a Pandemic --- An amazingly lame piece of "work," but it still can provoke useful discussions. I'll probably use this in 434 in the fall. The author seems to have not had access to CRSP.
Base Rates and Uses of Stockpiled Anti-Virals --- contains references that support a 1/33 or 1/30 base rate and which argues that 1/100 is attainable with proper stockpiling. Useful data links.
Prophylaxis in Close Contact --- San Quentin Prison 1918, not as grim as you might guess.