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?

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This is a virtual certainty—but with implications that are highly uncertain.

We can be sure we're counting things differently. We don't know the real numbers in Mexico, and the total caseload elsewhere is measured in dozens, which is small enough that you would expect only very high kill rates to show.

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Most of the dissonance appears to be from Mexico, where the numbers are bigger and reporting apparently more chaotic. There are some suggestions that a combination of disorganization and politics has slowed or outright discouraged rapid identification and tracking of these cases once it was apparent something was afoot. Doctors in some hospitals are reporting more deaths than now noted, coverups, sloppy containment, and generally dire situations; others claim things are a bit more orderly.

And because Mexico had the bad luck to have its cases emerge in the midst of its flu season, the virus had generated a significant case load and quite a few serious cases before officials realized something more serious was occurring and started tracking in earnest.

All this affects the apparent significance of the numbers involved. Of the 110 million people in Mexico, 1,600 cases have been reported, with about 100 deaths—suggesting a mortality rate of 6 percent. This is almost certainly bad math, as the total case count almost certainly ignores thousands or tens of thousands of other cases that have taken milder courses like those in the United States. It's perfectly conceivable Mexico has actually had 10,000 or 100,000 cases—or even 1 million cases. If so, then the kill rate would be not 6 percent but 1 percent (given 10,000 cases) or 0.1 percent (given 100,000 cases). * If it's 1 million cases (quite possible if this thing really spreads easily) then the mortality rate is just 1 in 10,000. Meanwhile, because the United States is on high alert—and can take special note of people with recent travel to Mexico—it is probably picking up a fairly high percentage of its cases, including milder instances that would have gone unnoticed in Mexico a few weeks ago.

If it hasn't infected that many thousands of people in Mexico, on the other hand, that would suggest that, though it may be deadly, it doesn't spread as readily as we fear. To hear of multiple tourist groups coming down with the virus suggests it spreads like wildfire. But it also ignores the virtual certainty that many tourists and other travelers have been exposed without getting ill.

That's not to be too sanguine. For one thing, it's also possible that Mexico is missing, undercounting, or badly underreporting deaths. But if this virus really does spread rapidly, its kill rate is fairly low; and if its kill rate is anywhere near as high as the 100-out-of-1,600 suggests, then it doesn't spread very easily.

AP video: Obama addresses the swine flu outbreak.

The answer doubtless lies somewhere in between. The CDC team that went to Mexico on Friday hopes to gather better numbers; with luck, they will soon get a decent grip on how many have likely been exposed, how many of those were infected, and whether a secondary infection or other factor might account for the deaths. That information will help us decide whether Mexico can be viewed as a reliable predictor of what will happen elsewhere.

As Obama's team wisely stressed in its press conference on Sunday, pandemics, like the viruses that cause them, behave in dynamic and unpredictable ways. And just as every flu pandemic differs from every other, a given outbreak, like this one, can display different dynamics on Day 100 than on Day 30, which is roughly where we are now. The transmission dynamics change as more people in new places get it. And as the virus encounters new populations, new environments, and new bits of other flu bugs, it, too, willchange—for such is a virus's nature—possibly becoming deadlier or more transmissible, possibly becoming less so.