Why does the swine flu seem to be more deadly in Mexico?

Health and medicine explained.
April 27 2009 3:20 PM

Swine Flu and the Mexico Mystery   

Why does the swine flu seem to be more deadly in Mexico?

Click here to read more from Slate on the swine flu. 

Two weeks ago, no one had heard of this strain of swine flu. Now it's on every front page and almost every continent. Is this the deadly global pandemic we've been worrying about?

Certainly there's cause for sober concern, if not alarm. For of the two qualities vital to a nasty pandemic—to spread readily and to be deadly—this flu, a brand-new strain of swine flu, or H1N1, seems to possess the first: Evidence is strong that it spreads readily among humans. In that sense, it's an inversion of the bird flu. Bird flu terrifies infectious disease experts because it kills about half the humans who get it—but it has so far failed to develop the ability to jump easily from person to person.

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This swine flu, meanwhile, does seem to spread easily by airborne transmission. But how deadly is it? Despite the 100-plus deaths in Mexico, we don't really know. And that's why epidemiologists are working frantically to figure out the Mexico mystery: Why do the death rates there appear to be so much higher than those in the United States? In Mexico, it has reportedly killed about 100 of the 1,600 official suspected cases; elsewhere, it has appeared to take a far milder course, with zero deaths out of approximately 300 instances. There are several possible explanations for this discrepancy—any one, two, all, or none of these ideas could shed light on how deadly this virus might prove. In order of ascending likelihood for Mexico's higher mortality:

1) Perhaps population-level genetic differences render the U.S. population more resistant to this strain's effects than the Mexican population.

This suggestion has popped up on a few blogs. Does the more indigenous genetic makeup of many Mexicans make them more vulnerable? Though it's nice to see people think genetically, the genetic differences in question would have to be far wider than they are to explain the differences. This isn't like the smallpox situation of 500 years ago, when American Indians were decimated by a virus they'd never encountered while Europeans carried it easily because centuries of exposure had selected them for resistance. This strain of swine flu virus is apparently new to everyone—a combination of bird flu, seasonal human flu, and (predominantly) two kinds of swine flu, all in a form our bodies have never seen. There seems no reason any human population should resist its effects substantially better or worse than any other. We can probably put this "genetic vulnerability" explanation in a drawer.

2) We're really looking at two different viruses, but WHO and the CDC haven't picked up on it. 

This was a halfway plausible explanation before the full flu assays were done on significant numbers of both U.S. and Mexico cases. But the completed assays appear to show that the fatal Mexico cases match closely the 40-plus milder cases confirmed in the United States. It's possible, but highly unlikely, that a high proportion of the other 80-plus suspected swine flu deaths in Mexico will prove negative. At least for now, we can probably set this possibility aside as highly remote.

3) Some secondary health issue present in Mexico but not elsewhere—another bug common in the population or in hospitals—is combining with the swine flu to make it more deadly there. 

This is remains a distinct possibility. The CDC has tested samples from the fatal cases in Mexico for some possible secondary bugs and vulnerabilities and eliminated the most likely and the easiest to test for, but there's no blanket test for all such candidates. So it's still possible some other bug joined with the swine flu to claim most of the fatalities. It's also possible that Mexico City's air pollution sharpened the course of cases there.

4) Some difference in the way we're tracking and counting cases—a "surveillance difference"—is making the Mexico situation seem worse than it is and the U.S. situation seem better than it really is.