Unsurprisingly, the United States Travel Association has issued a statement asking unnamed officials to "resist inflammatory comments on Swine Flu." Comments like this one, from the Vice President, mayhaps?

I would tell members of my family -- and I have -- I wouldn't go anywhere in confined places now. It's not that you're going to Mexico, it's that when one person sneezes it goes all the way through the aircraft. I would not be, at this point, if they had another way of transportation, suggesting they ride the subway. If you're out in the middle of a field and someone sneezes, that's one thing. If you're in a closed aircraft or a closed container, a closed car, a closed classroom, it's a different thing.

His office quickly released a clarification. Biden, they said, did not say what he said, and if you heard him say the thing he didn't say, then that was incorrect. In related news, Biden and bank-run enthusiast Richard Burr will be having an old fashioned scare-the-hell-out-of-everyone contest at 3pm on the West Lawn. I hear Burr is bringing a mask.

But here's the thing: Biden may be right to induce a bit of panic. The United States Travel Association won't think so. It's their profit stream on the line, after all. But epidemiologists are probably quietly relieved by the Vice President's comments.

Last night, I spent some time with Arin Dutta's "The Effectiveness of Policies to Control a Human Influenza Pandemic: A Literature Review." The overview was prepared for the World Bank. In it, Dutta argues that the key variable in determining the spread of an infection is the "base reproduction rate," defined as the number of secondary infections produced by a primary infection. In other words, if one person has the flu, then on average, the base reproduction rate measures how many people will catch the flu from him. Lowering that rate is the key to pandemic response. And lowering that rate requires things like "forcing or urging people to limit contacts, encouraging hand washing or other personal hygiene, or promoting the use of facemasks." Some of it sounds trivial. But it matters. If the reproduction rate falls beneath 1, "the epidemic usually dies out." But even small variations in the rate can have large impacts, as you see in this chart:


"R" is the reproduction rate. And a small increase in R -- from 1.1 to 2.4 -- triggers a 600 percent jump in the attack rate. That's the problem with flu pandemics. They're dependent on collective action. An individual who takes non-essential plane flights -- even to Mexico, cause tickets are now cheap, and really, what are the chances? -- is making a rational decision based on individual probabilities. But if everyone does that, then we're hurtling towards a full-blown pandemic.

Which is why the absolute best case is that Joe Biden did something that's so effective that he looks really stupid. If people actually reduce social contact and cut down on air travel and stay home in response to a single cough, then it's much likelier that swine flu will quickly die out. If it does, we'll all feel a bit foolish over having taken those precautions and late night comics will make fun of Joe Biden and everyone will move on. If we don't, and R jumps up, then we could be dealing with a full blown pandemic and Biden's warning will come to be seen as, if anything, insufficiently alarmist.