Tuesday, April 28, 2009

A Tale of Two Flues

Ever since news of the outbreak of swine flu in Mexico there has been a whirlwind of coverage. One thing that is impossible to miss is the incredible range in the tone of reactions. US officials have been largely muted, while countries like Russia are screening every single plan that arrives from the Americas. Randall Munroe of xkcd did a good job, as usual, of capturing the internet's reaction:

There's actually a good reason for the disparity in reactions, however. It's a tale of two flues.

Spanish Flu Pandemic (1918)

An outbreak of influenza from March of 1918 to June of 1920 that swept the entire globe, infected nearly half of the entire human species, and killed up to 100 million people. This strain of the flu was unusual in that it caused an overreaction in people's immune system which led to a higher mortality rate among young, healthy people than among the very young and very old.

Swine Flu Scare (1976)

After an army recruit died and others were hospitalized with a new swine flu outbreak, President Ford took an aggressive stance and decided to immunize the entire American population. (I guess he'd never heard of herd immunity.) It turned out this particular strain of influenza wasn't really that dangerous, but the vaccine was. About a quarter of the American population was vaccinated before the program was halted, and by that time about 500 Americans had died from reactions to the vaccine.

The memory of the 1976 vaccine debacle explains in part the reluctance of the United States to go to extremes in responding to the current outbreak. In addition, there's the fact that cases in the US so far have been fairly mild. Finally, there's almost certainly political considerations with regards to America's open border with Mexico.

It's important to point out, however, that waiting for the next deadly flu strain is like waiting for the next big California earthquake. It's a question of when, not if.

Final thought: The words "pandemic" and "epidemic" are being slung around a lot these days. These words actually have definitions that go beyond "when a disease is really bad".

epidemic - An epidemic occurs when new cases of a certain disease occur in a given human population, during a given period, substantially exceed what is "expected," based on recent experience

This separates an epidemic from a disease that is endemic. Endemic diseases exist in a society perpetually but at a relatively low rate. Chicken pox is endemic in the United States.

pandemic - A pandemic is an epidemic of infectious disease that spreads through populations across a large region; for instance a continent, or even worldwide.

So I'd say - as a non-expert - that we've already got a swine flu epidemic in Mexico. It's not the season for flu and yet we've got thousands of sick people. If the swine flu spreads substantially across the United States then - between Mexico and the US - most of North America will be infected and we can start calling it a continent-wide pandemic.


Ju 4/28/2009 5:49 PM  

In 1918:

In large U.S cities, more than 10,000 deaths per week were attributed to the virus. It is estimated that as many as 50% of the population was infected, and ~1% died. To compare, in "normal" (interpandemic) years, it is estimated that between 10-20% of the population is infected, with a .008% mortality.

The fact the current 'swine flu' has shown to be contagious is alarming. So far the virus has shown to have a 6% to 6.3% mortality rate. It may not seem like much, but please consider the following: The deadly influenza panic in 1918 had a mortality rate of under 1%.

This virus went on to kill tens of thousands of healthy people a day in large cities and up to 100 million people world wide.

Viruses, like this strain of swine flu, kill their host by over-stimulating active immune systems that are robust and healthy. That is why the victims in Mexico were between the ages of 20 and 45.

Some have said that no one in the United States have died from the virus, so we need not worry. Experts say it is only a matter of time. The virus is not prevalent enough to reach statistical significance in the United States, with only a handful of confirmed cases. 93.7% of all Mexicans with the virus recovered.

More cause for worry: The 1918 virus started off 'mild' before it mutated into a raging storm. It also does not mean we will see millions of deaths. It is too early to draw sweeping conclusions. Nevertheless, there is potential for a disastrous pandemic. If 50% of Americans catch this flu in the next two years, and the mortality rate stays at 6.3%, we would witness 20+ million deaths.

This strain of virus is more potent and more deadly than the virus that hammered the world in 1918 and 1919. Viruses come in waves. There are striking similarities to this virus and the virus that killed up to 100 million people in 1918. The first wave is historically more mild than the later waves.

In addition to this virus becoming more severe, it is mutating faster than previous virus that we have seen. In addition, this virus is nothing like we have ever seen before because it combines features from viruses natural in different parts of the globe. We are in uncharted territory.

If it follows the same path as the 1918 flu, we will see very damaging results. However, we must remember we are a global society now and the virus can spread quicker than we have ever witnessed in history. This is very concerning especially since the drugs we have now seem resistant.

While there have been no deaths in America, it is shadowed by the fact the common variable among the deaths seem to be age. While most American cases have involved the very young and very old (under 10 and over 50) the Mexican cases that ended fatally involved the robust and healthy (over 20 and under 45).

This virus kills the host by over-stimulating the immune system. The term that is used when the immune system over reacts is called a Cytokine Storm. It is usually fatal. During this “Storm” over 150 inflammatory mediators are released. This would account for the high mortality rate in 1918-19.


the Stormin Mormon 4/29/2009 8:40 AM  


Thanks for the comments! Where are you getting your info about the current flu strain, e.g. the rate of mutation?

The one bright side you didn't mention is that so far anti-viral drugs in the US are said to have good effect. So even if transmission rate is very high, we may be able to keep the mortality rate much lower than what's been seen in Mexico.

Although you're right - there haven't been enough cases in the US to get a statistically valid sample of the mortality rate.

But we did have our first death. A 23-month old child in Texas is the first confirmed death from the flu.

To keep it in perspective, we have thousands of deaths every year from the flu. So it's still too early to know how bad this is going to get.

One other thing to keep in mind, however, is that mortality isn't the only damage the flu can have. There's also the question of economic and political fallout. Mexico, for example, depends heavily on tourist dollars. Depending on how long the outbreak lasts, their economy may be hit significantly. Combine that with their already weak economy, the global recession, and the drug violence and you've got a recipe for disaster on our southern border.

Many nations are in similarly precarious positions world wide, and the flu is not o nly restricting global travel, but has also already led to absurd trade restrictions (e.g. on American pork) that may work to deepen the current recession.