No, We Shouldn’t Close The Border. Sigh.

In the wake of the swine flu outbreak, we have the inevitable calls for closing the borders with Mexico (h/t):

“A spokesman for Rep. Trent Franks, R-Ariz., said Wednesday night that Franks believes the border should be closed right now except in critical cases or situations involving emergency personnel.

Sen. John McCain, R-Ariz., said all options should be considered to end the crisis involving swine flu, “including closing the border if it would prevent further transmission of the deadly virus.”

In a twitter message early Wednesday, McCain wrote “I said to Napolitano, ‘We need to be prepared to close the border with Mexico if the swine flu outbreak escalates further.’””

Regrettably, this idiocy seems to be bipartisan:

“Rep. Eric Massa (D-N.Y.) said the border should be closed until the threat is resolved.

“The public needs to be aware of the serious threat of swine flu, and we need to close our borders to Mexico immediately and completely until this is resolved,” Massa said in a statement.”

Closing the border is a silly idea. For one thing, in case these Congresspeople hadn’t noticed, swine flu is already here. For another, it would cost a lot of money:

“One 2007 study by the Brookings Institution estimated, for example, that a 95% reduction in U.S. air travel would cost the economy $100 billion a year.”

And that’s just air travel. Shutting down the border with Mexico would mean shutting down all US-Mexican trade. That would do serious damage to both economies.

Besides all that, closing the border would be very unlikely to work, even if the border were a lot less porous than it is. The short explanation is: there are several factors that determine how effective you’d need your border closing to be in order to keep enough infected people out. The first is how infectious the disease is. Basically, you want your public health measures to bring the average number of people that a person with the disease you’re worried about infects below 1, since if every person who’s infected infects, on average, less than one person, the disease will die out. If you know how infectious the disease is, that tells you how successful your interventions will need to be to achieve that result.

If, in the absence of any public health measures (and any built-up immunity), an infected person will infect, on average, 1.1 people, then you don’t need your various measures to be all that perfect in order to contain the disease. If, on the other hand, an average person would infect, say, 6 people in the absence of public health measures and immunity, then your public health measures have to work very, very well if you want to contain an epidemic.

(One way to think of it is this: once people are in your country, they will start infecting people, and if the average number of people that each person infects is over 1, the number of infected people will begin to increase exponentially until enough people are resistant to the disease, or dead. Your border control efforts, regrettably, will probably not increase exponentially. If the average number of people that each person infects is in the normal range for the flu — 1.5-4 — and the disease has a short incubation period, which the flu does, this means that in fairly short order, the number of people infected within your country will begin to swamp the number of people you’re keeping out.)

Unfortunately, the swine flu is pretty infectious, partly because no one has partial immunity to it. It also spreads fast. For that reason, you’d need a border closing, or any kind of movement restrictions, to be very effective if you wanted to block it. One thing that helps make movement restrictions effective is having a readily sealable border — the sort that a very small island nation might have. Obviously, the US does not have such borders.

It also helps if the disease you’re trying to keep out has the right sort of profile, and specifically, if it produces symptoms before it makes people infectious. If you have a disease like that, you can screen people for the symptoms, the way Tokyo is trying to do by using heat sensors in its airport to spot people with fevers. If you get all the people with symptoms, you will get all the infectious people, who are the ones you need to spot if you’re trying to contain an epidemic. You can also hope that people who develop symptoms will realize that they’re sick and decide not to travel.

Unfortunately, the flu is infectious for about a day before people develop symptoms. That means that any attempt to screen people at the border will not work. (So much for those heat sensors.) You’d have to keep everyone out, period. We can’t do that even without an influenza epidemic; I have no idea why anyone thinks we would suddenly be able to do it now.

Earlier today, Ezra linked to a World Bank review of the literature on containing pandemic flu. It explains the pros and cons of various measures, and estimates of their likely effect, quite well (if a bit wonkily.) The discussion of travel restrictions starts on p. 30, though some of the terminology is defined earlier. The takeaway message is that even very effective border controls, including shutting down almost all air traffic, would have very little effect.

This is worth bearing in mind more generally. Whenever a new disease pops up, people start talking about closing borders, quarantines, and so forth. It’s generally a good idea to isolate people who actually have the disease, to urge people to voluntarily refrain from hanging out in crowded places, and so forth. But quarantining people who might have been exposed means violating their civil liberties, and shutting down borders means taking a serious economic hit. In both cases, you’d need some assurance that taking these steps will actually achieve something that makes those costs worthwhile.

If a disease (a) is pretty infectious and (b) makes people infectious before they develop any symptoms, then neither border controls nor quarantines are likely to work, absent very special circumstances (e.g., someone develops a very serious disease like ebola on an airplane, where you really can completely control the airplane’s exits.)

If you hear someone talking about shutting down borders or quarantining people who are not already ill, if the disease they’re worried about is reasonably infectious, and if people with that disease are infectious before they’re symptomatic, and if you do not live in a small and remote island nation, then it’s a pretty safe bet that that person don’t know what s/he’s talking about. We should expect better from our Senators and Representatives.

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